We have a page where we're assigning available users to a list. In other pages, we use two lists, side-by-side, and use assignment buttons between them to move users from available to assigned. However, due to space constraints, it's been suggested that we use a combo box (with type ahead) such that when the user selects an entry in the combo box, the entry is automatically added to the list.
One member of the team wants to include an add button so that only when the add button is clicked is the entry added.
Which is the better pattern from a UI/UX perspective? What are the pros and cons of either solution?