Are you getting an error message? Have you tried logging in to groupme.com first and then clicking the link in the post to connect?
It tells me the password is invalid. When logging in directly I can't use the user and pw, I need to click Sign in with Microsoft. Once I'm in, clicking the link in the post still prompts me to log in.
The weird thing is that from within groupme I can edit my password, but it still won't take it when I try to connect.
I would try to convert my login type to use a regular password but I don't see a way to do that