Good luck at your meeting, Feelyunicorn.
I was also very nervous at my first poly group discussion. I drove an hour and a half by myself to a place where I knew absolutely no one. (That alone made me nervous even without the confusion of not knowing what to expect from a poly group.)
Everyone was very nice and welcoming. I did not feel uncomfortable at all.
But I also didn't feel like I fit in too well. Everyone was a bit clique-y. Everyone seemed totally experienced with poly and totally sure they were poly, rather than new and just questioning, like me.
Also, everyone had at least one partner (even if they didn't bring the partners with them to the meeting). I definitely felt like I was the only person there who was un-partnered and unsure of what I was looking for in a relationship (or whether I wanted a relationship at all).
However, there were no couples making out! I don't think you need to worry about that.
Ultimately, while I've had no negative experiences at poly groups, I don't really feel that the poly community is "my" community.
Feelyunicorn, the best advice I would give you is just to go to see what it's like. Don't expect too much, but go expecting to feel excluded either.
Also: don't hit on anyone or flirt with anyone at your first meeting. I don't know about the Brazilian poly community, but the groups I know are very clear about making discussion groups a safe place where people (especially women) don't have to worry about unwanted attention. Nor should you assume that all poly people are available.
After you get comfortable with the group, you can be more open about yourself and your situation. You might be able to raise the issues of finding support for the single-by-choice and for a more casual-sex-positive environment.
I personally noticed that those two aspects of my own life (single by choice, and open to pursuing more casual-type dating) are not much addressed in my local poly community.
Good luck, and tell us how it goes! I didn't even know there was any type of poly community in Brazil.
Single, straight, female, solo, non-monogamous.