I'm not sure I understand your situation.
You are dating one woman. You are thinking about dating another.
What is the difference between dating these two women, and being in the poly circle? Are you talking about moving in? Sharing expenses? Communal living, sharing assets? Getting involved with the other two people in some way? Or just dating these two women, and being a friend to the rest of the household?
There are no protocols. We all do this sort of thing differently. That's why I'm not really sure what you mean by getting involved with this circle.