Yet there are two down-sides of this approach: 1) Users must apply themselves and 2) the user group leader must process the applications manually. This might be suitable for small, exclusive groups, but once you want to target a larger group of users, you'll see yourself wasting a lot of time manually assigning users. Burning Board 4.1 closes this gap with the ability to define automatic user group assignments:
You can define a lot of different criteria which must be fulfilled to assign the user to the configured user group. In the simplest case you want to add users to a user group with higher privileges once they're registered for a defined period of time or if they used a specific email address, e.g. a corporate email address. You can even build up multiple levels of assignments, for example a user will be promoted to group B if he/she matches the criteria and is already member of group A. In addition you can exclude users which are banned or not yet approved.
In some cases the generic attributes of a user (days registered, account status, etc) are not enough and you want to bind the assignment based upon their contribution towards your community. Besides the amount of posts and received likes, you can bind this to the accumulated activity points. The activity points are a powerful unit to measure a user's activity since they're granted for various activities within your community and the individual activity points per action can be freely configured.
The last tab is the most specific one as it focuses directly on the user's private details and is intended to provide a fine-grained filter. This list is not static as it reflects the actual profile fields available to your users, any custom user profile field will show up (requires field to be searchable).
Last but not least I would like to point out, that almost all conditions are non-exclusive and can be freely combined. The only exception are special attributes, e.g. checking both "Banned" and "Not Banned" will yield an error, if you want the assignment to be processed regardless of the banned state, simply select neither.