If you turn off "User Searching By Full Name" in "General Configuration" you can no longer add users via the Project Member Roles interface

Why would you do this? Well, it's related to https://jira.atlassian.com/browse/JRA-7776

Ideally, you would be able to define a project scope for a user-picker, but you can't. If you have a group of users who should not see each other in your instance, your best option (that I can tell) to manage this is:

(1) Turn off the auto-pop on the user-picker fields via the General Configuration interface and
(2) Limit Browse Users to the group who should see ALL users in the system

When you do this, though, adding users becomes very lame (ideally, the auto-pop would be definable per project), or in the case of defining member roles on a project, broken.

There is a work-around, which is to add a member to a project through his/ her user profile, but this takes twice as long.

I would assume that if JIRA-7776 were addressed, this would either no longer be a problem or would be addressed at the same time.