The much requested feature was announced by +Brian Glick, a product manager for +Google+, in this post in which he said:
"...today, we’re rolling out a frequently-requested feature: the ability to search for members in your community. This will make it easier to manage a growing member list, especially if you’re moderating a large community."Since Communities launched back in early December of last year, it has been difficult and cumbersome to find specific users. If a member posted a spam comment on a post, community moderators would be able to report and delete the comment, but with the (previous) lack of a search bar, it was a pain scrolling through potentially thousands of users just to locate and ban the offending member.
Now that there is a community member-wide search engine, I can easily type in the head honcho's name, +Vic Gundotra, and easily see that he is a member of our very own Google+ Updates community.
It has been a long while coming (or what seems like it), and maybe it should have been there from launch but we can't complain now it appears that there is a constantly trickling stream of updates for Communities.
Make sure to keep submitting feedback by holding CTRL + @ (Chromebooks & PCs) on any page on Google+ that doesn't already have the tool hiding away in the cogwheel menu, as the Google+ team are always listening to user comments, suggestions and feature requests.
Source: Brian Glick