How to grant access to pages via codes or passwords?

Hi, I'm curious if anyone knows of a plugin or other solution for granting access to pages via a code? It would be similar to the WP password protect option, but:
1. I need to set multiple codes or passwords to access the same pages/content. That way different groups can access, and their code can be expired, without it affecting other groups.
2. I'd prefer to be able to manage codes for all protected content in one place, instead of individual passwords on each page.

Basically I've done membership sites. I know how to do that. But this client wants to give easy access to an entire group, without having to have a user account for each and every user, or even for the entire group. Just one code per group.

Has anyone seen anything like this?