I was considering moving a few sites to wordpress that have in excess of 30,000 registered members. Technically there will only be about 150 users concurrently on the site and most of those on the forum (not wordpress.)
The question is one of scalability. I've seen posts on a site in excess of 100,000 and the site works fine. I've heard that once the number of users goes up, WordPress isn't designed to handle that many users due to multiple joins. I've noticed some of this with the users/usermeta relationship in addition to other queries that make me believe table locking will occur. Actual users needing to manipulate WP such as posting at one time will be maybe 15. Actually number of users logged in if we use WP as a registration system could be more like 50-100, although many will either be viewing a post or just logged in and elsewhere such as in the forum.
So is WordPress capable of having tens of thousands of user accounts and managing itself reasonably well? Should we stick with a separate membership database that adds accounts to WordPress as needed by users who login separately to WordPress to create posts?