You don't have to use a link to invite people, if people have joined because you tell them about this forum they can put your name down as a referral, depending on whether they know your username here, of course.
Jenna, the prizes don't necessarily have to be anything big and you might get donations from some of the other members here if you think about it a bit. There's also an add-on for a forum store whereby members earn forum $$ for posts which they can bank or use to buy items from the store such as changing the look of their screen names plus any items you might want to add.