Groups, fields and entities
This screencast shows how to use non-node entities (or entity bundles) as groups and group content. Normally you are probably satisfied with using nodes for this, but in some circumstances it makes sense to utilize other entities.
The best/easiest way of turning other entity bundles in to groups, or group content, is to visit the Organic groups field settings page under the configuration option in the administration menu. New fields are added by selecting a bundle, selecting an OG field, and hit the add field button. Fields can be removed either from this admin page, or from the field manage page for the relevant bundle.
(For the record: "bundles" are types of an entity – content types are node type bundles, and tag terms are taxonomy terms belonging to the bundle tags. Some entity types – such as user accounts – normally only have one bundle.)
If you try to make taxonomy terms into groups, you'll want to follow this issue: http://drupal.org/node/1076024#comment-4620716