Magic Forums

While looking over the many individual covens here, I noticed that many covens list other covens as "sister covens" (I hope this sentence doesn't confuse anyone!). I assume that a Sister Coven would be an associated group, but how involved are these covens? i.e. Do they typically collaborate? Are members welcomed into sister covens? Etc.
I did not see any forum posts that answered my questions after an admittedly quick search, so I thought that I'd create one.