How to refresh doc groups for logged-in user?

I’m adding new group access for logged-in users after they make a purchase. The notification comes in via PayPal IPN at which point I add the user to the group, then send them off to the home page for that group. Unfortunately, at that point the user gets the 403 page and has to logout then back in to get access. Seems like I saw somewhere in the core that groups for a user are cached when they login. If that is the case, does anyone know if there is a proper way to flush or update that cache? Or perhaps a better way to accomplish what I’m up to?

Many Thanks

Rick