Add following TwAPI endpoint.

lib/pleroma/user.ex

@@ -1,6 +1,7 @@
 defmodule Pleroma.User do
   use Ecto.Schema
-  alias Pleroma.User
+  import Ecto.Changeset
+  alias Pleroma.{Repo, User}
 
   schema "users" do
     field :bio, :string
@@ -26,4 +27,20 @@ defmodule Pleroma.User do
   def ap_followers(%User{} = user) do
     "#{ap_id(user)}/followers"
   end
+
+  def follow_changeset(struct, params \\ %{}) do
+    struct
+    |> cast(params, [:following])
+    |> validate_required([:following])
+  end
+
+  def follow(%User{} = follower, %User{} = followed) do
+    ap_followers = User.ap_followers(followed)
+    following = [ap_followers | follower.following]
+    |> Enum.uniq
+
+    follower
+    |> follow_changeset(%{following: following})
+    |> Repo.update
+  end
 end

lib/pleroma/web/router.ex

@@ -31,5 +31,6 @@ defmodule Pleroma.Web.Router do
     post "/account/verify_credentials.json", TwitterAPI.Controller, :verify_credentials
     post "/statuses/update.json", TwitterAPI.Controller, :status_update
     get "/statuses/friends_timeline.json", TwitterAPI.Controller, :friends_timeline
+    post "/friendships/create.json", TwitterAPI.Controller, :follow
   end
 end

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")