User Group trouble

[musik]

I'm trying to make sense of user groups and the promotion settings.

Basically, when a user has confirmed their account they become a part of Medlemmar, and the way it is now they'll be moved to Medlemmar 3 once they've reached 50 posts - and that's been set up by the admin group before the current one.

What we now want is to have a middle step where it would look something like this:
0-10 posts = Medlemmar
10-50 = Medlemmar 2
50 = Medlemmar 3

I've been pulling my hair trying to make sense of it but I just can't, any ideas?

Thanks in advance.