Fix form url and routes path

app/views/groups/_new_group_member.html.haml

-= form_for @team_member, as: :team_member, url: team_members_group_path(@group, @team_member) do |f|
+= form_for @team_member, as: :team_member, url: team_members_group_path(@group) do |f|
   %fieldset
     %legend= "New Team member(s) for projects in #{@group.name}"
 

config/routes.rb

@@ -103,7 +103,7 @@ Gitlab::Application.routes.draw do
       get :merge_requests
       get :search
       get :people
-      put :team_members
+      post :team_members
     end
   end