Well, I didn't do a great deal of approving or rejecting people applying to join, to be honest, but I also followed Lars' process. If it looks suspicious, wait and see, and check back a while later.
I have been much more involved in policing posts on the group, than checking members.
The process for that was as follows (using a recent example):
In the group, someone posts a link to a somewhat circular economy/sustainability focused app which has no relation to open source.
I check the link to make sure that it really has no element of the open source idea
I delete the post
I send the person who posted it a message:
"Hi, thanks for your interest and for posting in the OSCEdays group - just to let you know that we're trying to maintain a good signal-to-noise ratio and really focus on projects and ideas which encompass ideas of both open source and circular economy, rather than sustainability and waste-reduction more generally.
The app you linked to is useful, and more people should know about it, but as it isn't open source it's not too relevant to the OSCEdays conversation, so I'm deleting the post.
I hope you understand and please do keep spreading good sustainable initiatives elsewhere, and if you come across projects at the intersection of OS & CE, we'd love to hear about them.
and sometimes when it was clear someone is just spamming, I also sent them a message, but more direct:
I deleted your post from the OSCEdays group because it showed no relevance at all to Open Source or Circular Economy.
please be thoughtful about posting in groups and don't spam communities with irrelevant events!