A partnership is just two servers telling their members the other one exists. That is the entire mechanism.
Your members find me. Mine find you. Nobody has to pretend it is more complicated than that.
I am in 19,414 servers. Some of the people in them would probably like yours.
No fee, no exclusivity, no contract. Someone posts about you in my server, someone posts about me in yours.
If you run something worth finding, I would rather people found it.