I, like you, am new to all of this. But it sounds like you want the same things that I do. Not just casual play, but real, committed, long-term relationships with more than one partner.
Based on the reading I've done on this forum and other places, you'll find that there are as many different kinds of polyamorous relationships as there are polyamorous people. For those of us that are looking for commitments from all partners, there are certainly possibilities.
For example, I recently met two people who are looking for a third person to join their marriage, creating a relationship structure that I've heard people refer to as a "triad." That is, all three members are committed to each other as equal partners in the relationship.
Any kind of polyamorous relationship can be "closed" (no play/relationships with outsiders) or "open." Personally, I'm looking for a "closed" polyamorous relationship. That's not to say that there might be another person that joins the group in the future, but simply that there aren't any casual encounters with members outside the group. I would be comfortable in a larger group as well, and would love exploring a "group marriage" someday, that involved five or six or more people. But that's just me.
Basically, to sum this all up, I'll just pass on the wisdom I've gleaned from others:
1. As I said, there are many kinds of polyamorous relationships.
And 2. It's very important to be communicative with others about what kind of relationship you want.