when pressing action to add members to a role, you see all users who aren't members of that role with button to select one. Then the selector GUI closes.

Could:

1) have a "Close" button at the bottom, with a checkbox saying "Close with selection" next to it (selected by default)

or

2) have a checkbox column and an "Add selected users" button at the bottom of the screen

In either way the user could quickly add multiple users without opening-closing that GUI again and again and again (very tiring)