What’s a good way to put authors into groups

I have 7 companies contributing to one blog, and each company has 3 authors. I’d like to show all posts by all authors of one company. Is that possible?

To do this literally (put authors into groups) best way would probably be to create custom taxonomy for users. See custom user taxonomies in WordPress for reference.

However the end result you are looking for does not really deal with grouping users – it deals with grouping posts.

Registering custom taxonomy (“company”) for posts would be more simple and straightforward.

See register_taxonomy() in Codex.