You can treat BUG registrations like any other registrations, and assign consecutive shooter numbers by default.  As you can see in the top example, when I said to register Jane in BUG division as well she was assigned shooter numbers 34 and 35.

In this case, BUG registrations are 'interleaved' with regular registrations.

With our club, people pay a separate registration for BUG stages.  So I find it easier to have a separate set of shooter numbers for BUG stages.

If I start numbering BUG shooters at 201, for example, shooter 201 will correspond to the first guy on my BUG sign-in page, 202 to the second, etc.

In this case, I need to enter a 'seed' number at which to begin numbering BUG registrations.  With a new match, that defaults to 201.  If you typically have more than 200 registrations for a match, you could set it higher.

I've registered several shooters in the match at the right.  Some are only in a regular division, some are also in BUG.  The 'seed' number (which is the shooter number for the next BUG registration) is currently 204.