Your contact's Overview screen contains a Tag Groups section in the Know Your Client area for the contact. Below is an example of this section:
This area will display any Tag Groups that the contact is currently a member in.
The Add button in the upper right corner will allow you to include the contact in additional Tag Groups:
The Actions menu for Tag Groups displayed on a contact's Overview will allow you to remove the contact from that Tag Group or to view all of that Tag Group's members: