Can users change their usergroup without assistance of admins?

[amfor]

Is it possible to allow users of my forum to change their usergroup by entering special code received from administrator of forum?

They can receive this special code from administators of forum by private mail or download it somewhere else from Internet. The user should enter the code in his account setup and upgrade his usergroup manually. The code will allow users to upgrade their usergroup to another and get special privileges like unlimited download or something like this.

Whether there is a similar hack? Unfortunately can't find nothing on vbulletin.org but know that other engines allow this feature.

[squidsk]

Two options, if you want it linked to something, like payment, you can create a subscription on your site which can be setup to automatically put users in a desired group. You can also make some groups publicly accessible (usergroup permission), which means that users can apply to join the group and must be approved by either an administrator or someone chosen by administrators to be the group leader.