A quick look through the forum rules/information didn't provide an answer but I believe it is just based on the number of posts by the user. I can't tell you for sure what the numbers are but I can say there is not someone sitting behind a curtain grading members. I'm guessing at fifty posts you become a member and at one hundred, a senior member.
I used to administer a forum on a similar platform. The administrator set the quantity of posts required to move up the ladder, and created the names for each level. Being in Hawaii, we had cutesy names ... Malahini, Da Kine, Kamaaina, Punatic. I much prefer Junior and Senior Member. And, yes, the idea was to get people to post/participate more. I can't feature posting solely to move up in the rankings but maybe some do.
When this forum first started they paid us early joiners to post. To get it going. I can't remember the number of posts required to receive a check but I did receive one.