You can quickly view a group's membership by using the Search option on the People >> Subscribers >> Active page (or any other list of subscribers.)
If you'd like to then remove selected subscribers from that list, simply select the appropriate usernames and then choose Remove from group from the menu at the top of the list.
Please note: Removing a subscriber from a group doesn't delete their account. To delete a subscriber from all of Omnilert, click Delete in the menu! However, be careful, as deleting is permanent!