this is standard procedure for plaintiff's class action firms. each hopes to get the "lead plaintiff" for the class (at best) or to divvy up the pie with the firm that gets the lead plaintiff by contributing the other plaintiffs they have signed up (at worst).
you don't have to join the suit now, but if a class is certified, you may not have much of a practical choice. it's theoretically possible to "opt out" of the class, but that can reduce your chance of recovery unless you're ready to pay for your own counsel.