WP User Groups allows users to be categorized using custom taxonomies & terms.

• “Groups” & “Types” are created by default, and can be overridden
• More user group types can be registered with custom arguments
• Edit users and set their relationships
• Bulk edit many users to quickly assign several at once
• Filter the users list to see which users are in what groups
• Not destructive data storage (plugin can be enabled & disabled without damage)
• Works great with all WP User & Term plugins (see below)

Also checkout

• WP Chosen
• WP Pretty Filters
• WP Media Categories
• WP Term Order
• WP Term Authors
• WP Term Colors
• WP Term Icons
• WP Term Visibility
• WP User Activity
• WP User Avatars
• WP User Profiles