How about offering them free accounts for a fixed period, like 3/6/12 months? That way you could reward their support without having to carry a bunch of free us

How about offering them free accounts for a fixed period, like 3/6/12 months? That way you could reward their support without having to carry a bunch of free users in perpetuity. If they like your product they probably wouldn't mind paying for it, and this explicitly communicates the value you're offering. Also, for any given product I'd expect a small group of very active testers, and a larger group of semi-active to inactive ones.