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