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They will search deep in your soul to determine your core nature. This is the deciding factor. It's not something you can change. Some people are better suited to become incubus, some should be succubus freelancers, some may be better as a different kind of demon or even an angel, but they feel an attraction to the idea of following a path like this. No one can tell before looking if you can join or not. The Collective is very meta in nature, viewing history from a place with no chronological time, this abstract understanding of the world is also rather special and cannot be achieved by everyone.
If you are accepted, they will tell you so. If not, they will refer you to a suitable guide from a different place.
There were people whom they said to let into the practice group but who couldn't join the Collective in the end, we had one freelancer and one dragon earlier. They stayed a while, then left after learning some practices.