Hi,

I like JoomDOC very much for hiding documents from users and just make them available forspecial groups.
As stated on the website you are open for feature quests:

Change the way Users and groups are bound.
At the moment you are only using the jos_joomdoc_groups table.
You could introduce a table like jos_joomdoc_usergroups which maps the joomdoc group to the joomla users and adds a start and end date for the group membership.
This would offer the admin to release files for those people only in the specified period.
Unfortunatly this would maybe be a not too small change to the backend UI as the current implementation of adding users to groups will probably not fit to the new workflow.
For the frontend on dowloading files and accessing categories the only change would be to implement an additional date check.

I hope you are interested in my proposal.