We have added a user pick list as a custom field to one of our layouts. Now when I add a new user to a project that uses this layout the  user will not show up in the user pick list.

The only way I found to include the new user to the list is to open the task layout in edit mode, edit the user pick list and click on "Select all".

Is there an option that I am overlooking that would automatically add all new users to the users pick list?

Any hint is appreciated.