XD I love the mods of Harry Potter fandom communities, you're all always so nice and eager to affiliate :)
Well, what we usually do is I put a link to your community in my community's userinfo (I also add it to a list on the layout but you don't have to do that) and vice versa and the idea is that Harry Potter fans who join your community will see your affiliates and check them out as well. It's a great way to get more members :)
Making a post about your new affiliate is also optional, I usually do a bunch in one go, just so current members hear about it (cause they wouldn't regularly check the userinfo).
^___^ I'm so happy for a new affiliate (that isn't my other community XD)! Thanks so much -trundles off to add you to the list-