I'm a little slow on the uptake. I just picked up on the fact that you wanted the groups by AccountLetter. If I'm correctly interpreting that, the attached will do it. Because of the difficulty of incrementing on 2 text fields, I added an autonumber field called UserID. You don't have to use it for anything else. Then I created a query called "qryIncrement", and an update query based on that; "qryUpdateGroup". All you have to do is add that field in your table, import these 2 queries and run the update query. All on a backup of course!
I'm not really sure this is the best solution; that would depend on the overall situation. Frankly, it feels a little "quick and dirty" to me, even though I came up with it. Alternatives would include a function that ran for new records, filling in any blanks left by deletions. That way you wouldn't have to run this all the time, and people would stay in the same group (of course that may not be a concern).
Anyway, see if these help any.