The posts column number means "they make this many posts and they get upgraded"; you'll want to reset Users back to the 0 it was by default, unless you plan to have it set that users are in a temporary group until they hit a specified amount of posts.
If you wanted to make that First 100 group be where people go after making 100 posts, you'd put 100 in the posts column.
As for adding a new group and existing ones being cleared out, the create group routine is extremely basic, there isn't anything in there that would change any existing groups, feel free to take a look at /admin/docreategroup.php; the only thing I can think of is if the auto group assignment of "1 post" in the users group could be throwing things off.
