I think the titles work something along these lines:

"Newcomer" = 1 - 50 posts
"Full Member" = 51 - 200 posts
"Senior Member" = 201+ posts

They change automatically as you add more posts. I might be off with the exact numbers, so don't quote me on them. It doesn't help that I can't actually remember the changes happening, I would suddenly notice that it had happened some time after the fact. This doesn't include OFFICIAL SUPPORTER. That was, and is, a decision that only the staff at Captain Sim can make. ;)