Yes,
It's wonky though, they have to first post it comment in your community, then you can add them there. In the web ui it shows up after clicking the 3 dots on the comment/post
Thanks for the hint. I thought, I can add members to mods if their have made her first post. Thanks to the info, I just noticed that this doesn't apply to posts, but to comments. Is it just me or is that a little weird? It doesn't matter, I know how it's done now and I'm happy that I learned something again.