A lot of groups seem to keep themselves to themselves and it can be very difficult to join groups in a sisterhood as for example people on this group/site may not agree with everything that is done or said on another site. Also some groups may only like one type of pagan faith in their group, eg the wiccan site you have suggested may only like wiccans to join their group.
All I would say is exercise caution when requesting sister group positioning. Some site runners dont take kindly to it. Approach it in the same way you would joining a covern.
Im sure I am preaching to the choir so to speak but Ive seen this attempted before and it didnt end well xxx