Fix ergonomy of users/groups administration screens

In the following scenario involving 3 groups, ergonomy could be improved.

How to reproduce:

• create 3 groups A, B, Z
• in A, add the member Z
• when clicking again on the "add new member" button, it toggles on/off the display of the member addition, while "add" semantics is expected
• Then, go back to main screen of users/groups, and select group B
• Click on "Add new members" button, and Z is presented automatically

Request to enhance the ergonomy of these screens, which can be handled by non-technical people, e.g. powerusers.