Vendors for Pre-Launch Waitlists with Referrals

Hi everyone!

Wanted to see if anyone here has experience or knowledge of vendors that offer services for pre-launch waitlists with referral systems?

Setting up a pre-launch landing page for a new startup that will feature a waitlist. Would love to be able to incorporate a referral system into that as well.

Wondering if anyone knows what the top vendors in this space would be, or has recommendations/experience working with any providers?

Many Thanks!

I've used before for this