Rails Activeadmin – custom association select box

Yes, that is possible. I assume you want to use a DropDown list box for members to select a user from User model.

form do |f|
  f.inputs do
    f.input :user_id, :label => 'Member', :as => :select, :collection => User.all.map{|u| ["#{u.last_name}, #{u.first_name}", u.id]}
    f.input :description
  end
end