User Confusion

Hi,

I have a confusing situation with users. Now the users who joined at the very beginning, a year ago, seem to be members of the main site and the network, similarly with those that have joined in the last 2 months or so. But I have the main bulk of my users who joined somewhere in between these times who are part of the network but are not part of the main site. I first noticed this when I installed the newsletter plugin, I was trying to find a way to add them to the newsletter. Now I have a situation where I want to add one of these users to the main site as an author, but I can't. They don't seem to be a user of the main site, but when I try to add them as a new user, it says that the user already exists. So what can I do? Would there be a way of getting the users from the network and adding them to the main site?

Thanks!