Okay, well here's my advice, based on what you've described.
It sounds like you've encountered a poly group that is having some problems. If the man at the center has three girlfriends who are all dissatisfied in some way with their relationship with him, it's very unlikely that he can "fix" things by bringing another person in. Some people have an impulse: "Relationship broken, add more people," which is generally bad news.
If, however, he recognizes that this structure (one person being primary to three, all of whom have no other relationship aside from their connections with each other) is a little unstable, and he is encouraging his girlfriends to develop other relationships so they don't only look to him for emotional support, that could make sense.
It sounds like you already have/want relationships with the two women you are dating/about to date. That's great! And it's good that their poly group is welcoming of you. If you are also invited to participate in group sex, well, think about whether or not that's something you want. Nothing wrong with it! But you should only go for it if you have the desire, if you feel that all physical safety considerations have been met, and if you have the emotional constitution for it. It can be fun and fulfilling, and it can also be hard, depending on how you bond with people, how you feel about casual sex, and why you're going for it
My word of caution, though... be wary about any situation that presumes that you will have a relationship of some kind with your partner's partner. That sort of thing can only grow organically. If someone really expects you to develop some sort of bond with someone you don't know well, it suggests a degree of naivete about the way human hearts work. You could hit the jackpot, and it could work out great, but it could mean drama.
As far dating someone when you know that the only role available to you is secondary... that's something I can do, and have done, and do. But you have to know your own heart, know what you want when you get into a relationship, and know exactly what they mean by "secondary," because different people mean different things. You'll want to find out what sort of time commitment, what sort of emotional availability, these two women foresee that they could have with a secondary relationship.
So I guess my best advice is to ask a lot of questions. Get to know how these different people relate to each other, what they expect of each other, where their expectations aren't being met. Date slowly. Jump into bed, if you like, but let your emotional bonds move as slow as you can, because you want to make sure that you manage your own expectations.
And if this group of people are very sex-driven, and sex-focused, my advice stays the same. Not all poly folks are interested in group sex, casual sex, sex parties, kink culture, etc. But for those of us who ARE, we are just an tender hearted as the more conventional types, and my best advice remains this-- take care of your heart.