I haven't done a mountain bike race with categorizations before but the way it worked for road biking races was you declared your classification when you register. Typically I think you start at Beginner category if you are in your first few races and then if you place high up you move to the sport category, etc.
Oh yeah start as a beginner. I made huge mistake my very first mtn race by signing as a sport since I have been mtn riding for years, i thought that I was better than a beginner so I wanted some "competition". Big mistake. I didnt even get to ring my bell..... This will be my third year and I am still riding in the beginner cat.