Buy Into Usergroup module

[ZiNgA BuRgA]

Updated to v1.0.1 - adds the ability for admins to specify time limits for staying in the usergroup.
You need to reactivate this plugin before any changes appear.

The time limits allow, for example, users to buy access into a usergroup, but it will only last for a limited amount of time (if specified by the admin).
You can set the time limit when editing the usergroup in the AdminCP.

Appears to work here, however, I have not had the chance to test this extensively.