Good to hear things are working out for you!
Well, stuff that would be good to know about include;
1) measures against STDs and pregnancy, fluid bonding, how to deal with possible crisis situations;
2) time-management; how many date nights per week, where, what are you going then, what about overnight stays;
3) what is their current dating status; are there any others in the mix atm, how long have they been poly, is this their first actual polyship/first ever in this formation, how out they are/expect you to be;
4) how much and what kind of communication is needed in what comes to relationship milestones, sex, overnights, meeting someone new etc.
5) how much say can you ultimately expect to have in the relationship; do they have plans to re-locate, have children, do any other major life-altering decisions in the near future that could potentially affect you?
Not all of those need to be covered on the first group date, but if your relationship progresses, they will most likely come up at some point.
Me: bi female in my twenties