Guest Group

I am administering a Forum on someone else's website.  When a new user signs up, they're automatically in the "New" group.  Occasionally, however, I notice a user in the "Guest Group", which obviously doesn't have the same permissions, and no matter how many posts they make, they'll never move up, since it's not a ladder group.  My question is, how does someone end up in the "Guest Group," and how can I prevent that from happening, since we want all new users in the "New" group?  Sure... I can go into the admin section and change their group, but I still want to know how they ended up in the "Guest Group" to begin with.  Thanks.

I understand the Guest Group is for non-registered members.  These people who end up in the Guest Group have usernames and passwords.  If they didn't register, how'd they get a username and password?

No, the users were not suspended, and Guest Group is not the starting group.  "New" is.  I just created a new user as a test, and it is in the "New" group, as it should be.  Haven't made any modifications to the forum, other than changing the group these "Guests" were in to "New".  I'll wait and see if I get another message from the site owner about someone else having this problem.