You can't define user groups when you're using OTFE (On the fly encryption). Makes sense, because it's kind of self defeating when you're using a master password (because of OTFE) that's the same for all users and then add some user passwords that only seem to offer some security.

The best way to use OTFE and multiple users is by giving all users a separate login for Windows, that'll make TB (per default) use different locations for the message base for every user. This also is offering all users the possibility to user different master passwords.