You can group contacts in Centrium using:

1) Contact tags

Centrium uses simple but powerful tagging mechanism to group contacts. To add contact to a group (tag it) just go to contact's edit page and add the tags that you want.

You can add multiple tags to a contact. When adding a tag, Centrium will auto complete tag's name to avoid spelling mistakes.
You can filter contacts by tag on contact's list page. You can also add tags to multiple contacts using group operations from the "Actions" menu on contact's list.

2) Contact types

Another option to group contact's is to assign them a contact type. Contact type can be selected on contact's add and edit pages. 

You will be also able to filter contact's list by contact type.

To set up Centrium contact types - account admin can go to the settings page, available here: