diff --git a/synapse/federation/transport/client.py b/synapse/federation/transport/client.py
index 3beab47832..370f7ba78b 100644
--- a/synapse/federation/transport/client.py
+++ b/synapse/federation/transport/client.py
@@ -614,6 +614,19 @@ class TransportLayerClient(object):
             ignore_backoff=True,
         )
 
+    @log_function
+    def join_group(self, destination, group_id, user_id, content):
+        """Attempts to join a group
+        """
+        path = PREFIX + "/groups/%s/users/%s/join" % (group_id, user_id)
+
+        return self.client.post_json(
+            destination=destination,
+            path=path,
+            data=content,
+            ignore_backoff=True,
+        )
+
     @log_function
     def invite_to_group(self, destination, group_id, user_id, requester_user_id, content):
         """Invite a user to a group
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index b98e30459c..4c94d5a36c 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -803,6 +803,23 @@ class FederationGroupsAcceptInviteServlet(BaseFederationServlet):
         defer.returnValue((200, new_content))
 
 
+class FederationGroupsJoinServlet(BaseFederationServlet):
+    """Attempt to join a group
+    """
+    PATH = "/groups/(?P<group_id>[^/]*)/users/(?P<user_id>[^/]*)/join$"
+
+    @defer.inlineCallbacks
+    def on_POST(self, origin, content, query, group_id, user_id):
+        if get_domain_from_id(user_id) != origin:
+            raise SynapseError(403, "user_id doesn't match origin")
+
+        new_content = yield self.handler.join_group(
+            group_id, user_id, content,
+        )
+
+        defer.returnValue((200, new_content))
+
+
 class FederationGroupsRemoveUserServlet(BaseFederationServlet):
     """Leave or kick a user from the group
     """
@@ -1182,6 +1199,7 @@ GROUP_SERVER_SERVLET_CLASSES = (
     FederationGroupsInvitedUsersServlet,
     FederationGroupsInviteServlet,
     FederationGroupsAcceptInviteServlet,
+    FederationGroupsJoinServlet,
     FederationGroupsRemoveUserServlet,
     FederationGroupsSummaryRoomsServlet,
     FederationGroupsCategoriesServlet,
diff --git a/synapse/groups/groups_server.py b/synapse/groups/groups_server.py
index 70781e1854..77b6273e39 100644
--- a/synapse/groups/groups_server.py
+++ b/synapse/groups/groups_server.py
@@ -723,6 +723,51 @@ class GroupsServerHandler(object):
             "attestation": local_attestation,
         })
 
+    @defer.inlineCallbacks
+    def join_group(self, group_id, requester_user_id, content):
+        """User tries to join the group.
+
+        This will error if the group requires an invite/knock to join
+        """
+
+        yield self.check_group_is_ours(group_id, requester_user_id, and_exists=True)
+
+        group_info = yield self.store.get_group(
+            group_id,
+        )
+        if not group_info['is_joinable']:
+            raise SynapseError(403, "Group is not publicly joinable")
+
+        if not self.hs.is_mine_id(requester_user_id):
+            local_attestation = self.attestations.create_attestation(
+                group_id, requester_user_id,
+            )
+            remote_attestation = content["attestation"]
+
+            yield self.attestations.verify_attestation(
+                remote_attestation,
+                user_id=requester_user_id,
+                group_id=group_id,
+            )
+        else:
+            local_attestation = None
+            remote_attestation = None
+
+        is_public = _parse_visibility_from_contents(content)
+
+        yield self.store.add_user_to_group(
+            group_id, requester_user_id,
+            is_admin=False,
+            is_public=is_public,
+            local_attestation=local_attestation,
+            remote_attestation=remote_attestation,
+        )
+
+        defer.returnValue({
+            "state": "join",
+            "attestation": local_attestation,
+        })
+
     @defer.inlineCallbacks
     def knock(self, group_id, requester_user_id, content):
         """A user requests becoming a member of the group
diff --git a/synapse/handlers/groups_local.py b/synapse/handlers/groups_local.py
index 5f7b0ff305..977993e7d4 100644
--- a/synapse/handlers/groups_local.py
+++ b/synapse/handlers/groups_local.py
@@ -229,7 +229,45 @@ class GroupsLocalHandler(object):
     def join_group(self, group_id, user_id, content):
         """Request to join a group
         """
-        raise NotImplementedError()  # TODO
+        if self.is_mine_id(group_id):
+            yield self.groups_server_handler.join_group(
+                group_id, user_id, content
+            )
+            local_attestation = None
+            remote_attestation = None
+        else:
+            local_attestation = self.attestations.create_attestation(group_id, user_id)
+            content["attestation"] = local_attestation
+
+            res = yield self.transport_client.join_group(
+                get_domain_from_id(group_id), group_id, user_id, content,
+            )
+
+            remote_attestation = res["attestation"]
+
+            yield self.attestations.verify_attestation(
+                remote_attestation,
+                group_id=group_id,
+                user_id=user_id,
+                server_name=get_domain_from_id(group_id),
+            )
+
+        # TODO: Check that the group is public and we're being added publically
+        is_publicised = content.get("publicise", False)
+
+        token = yield self.store.register_user_group_membership(
+            group_id, user_id,
+            membership="join",
+            is_admin=False,
+            local_attestation=local_attestation,
+            remote_attestation=remote_attestation,
+            is_publicised=is_publicised,
+        )
+        self.notifier.on_new_event(
+            "groups_key", token, users=[user_id],
+        )
+
+        defer.returnValue({})
 
     @defer.inlineCallbacks
     def accept_invite(self, group_id, user_id, content):
diff --git a/synapse/storage/group_server.py b/synapse/storage/group_server.py
index db316a27ec..16c11a056f 100644
--- a/synapse/storage/group_server.py
+++ b/synapse/storage/group_server.py
@@ -48,19 +48,25 @@ class GroupServerStore(SQLBaseStore):
             desc="set_group_join_policy",
         )
 
+    @defer.inlineCallbacks
     def get_group(self, group_id):
-        return self._simple_select_one(
+        ret = yield self._simple_select_one(
             table="groups",
             keyvalues={
                 "group_id": group_id,
             },
             retcols=(
-                "name", "short_description", "long_description", "avatar_url", "is_public"
+                "name", "short_description", "long_description", "avatar_url", "is_public", "is_joinable",
             ),
             allow_none=True,
-            desc="is_user_in_group",
+            desc="get_group",
         )
 
+        if ret and 'is_joinable' in ret:
+            ret['is_joinable'] = bool(ret['is_joinable'])
+
+        defer.returnValue(ret)
+
     def get_users_in_group(self, group_id, include_private=False):
         # TODO: Pagination