Add following TwAPI endpoint.

lib/pleroma/web/twitter_api/twitter_api.ex

@@ -34,6 +34,14 @@ defmodule Pleroma.Web.TwitterAPI.TwitterAPI do
     |> activities_to_statuses
   end
 
+  def follow(%User{} = follower, followed_id) do
+    with %User{} = followed <- Repo.get(User, followed_id),
+         { :ok, follower } <- User.follow(follower, followed)
+    do
+      { :ok, follower, followed }
+    end
+  end
+
   defp activities_to_statuses(activities) do
     Enum.map(activities, fn(activity) ->
       actor = get_in(activity.data, ["actor"])

lib/pleroma/web/twitter_api/twitter_api_controller.ex

@@ -32,6 +32,15 @@ defmodule Pleroma.Web.TwitterAPI.Controller do
     |> json_reply(200, json)
   end
 
+  def follow(%{assigns: %{user: user}} = conn, %{ "user_id" => followed_id }) do
+    { :ok, _user, follower } = TwitterAPI.follow(user, followed_id)
+
+    response = follower |> UserRepresenter.to_json
+
+    conn
+    |> json_reply(200, response)
+  end
+
   defp json_reply(conn, status, json) do
     conn
     |> put_resp_content_type("application/json")