With this current approch a user can set the value of the radio button with any team even if he is not a memeber of it. From what i understand. A project should belong to a team. If you already have it like this just replace the first line in create with this:
@team = current_user.teams.find project_params[:team_id]
@project = @team.projects.build project_params
This will assure that team id exists and related to the user