I've encountered a bug where the admin user can not select people after the select people lightbox shows up. To reproduce the issue, please go to admin console -> Click on People -> Group Summary -> Select any Group -> Edit Members -> Next to Add Member, click on Select People link.
The UI formatting is wrong and js is not work, but the console log didn't show any error under IE7/8's Developer Tool. It is working fine under IE9, FF and Chrome. Is this an OOB issue? I've attached a screenshot for your reference.