OK, I used the Support Toolkit to resynchronize the Newly Registered Users and Registered Users groups. That seems to have fixed the problem, except everyone in the Newly Registered Users group is also in the Registered Users group. Is that how it should be?
I was going to use the script last night, but realized it was for an older version of BBpress. The database schema has changed a lot. The forum postings are now in the wp_posts table, along with everything else. Does anyone have an updated script?