UGP has been discontinued. Discover the reason and how to deal with it here: UGP Announcement
Guides to migrate to standard user picker or upgrade to UGP PRO are available as well.
UGP conserves the great JIRA user picker functionality and adds a twist to it: the users may be filtered to one or more groups or roles. What you get from it? First, a limited number of users to pick from and more important, the right people you need. Imagine you want to assign an issue to a tester and ask yourself why you should have another one on sight for that purpose!
User Group Picker is a customized custom field that helps on filtering users in order to be chosen from the right group(s) or role(s) that you previously specified in fields configuration. And not to worry, the autocomplete function of JIRA user picker is here, too.