I have an umbrella site where my users will be registering. I will be providing access to several other related sites to my members. SO my question is how do the sites need to be configured so that a user can have one single login for multiple sites?

Do the related sites have to be on the same DNN install or can they be separate? Any thoughts on this would be appreciated.

I know there has been some discussion on this in the past and it seems like this concept has changed a couple of times along the DNN development path. But finding anything using the search function on these forums is really difficult. I just get tired of searching and getting no results. Sometimes I even see a phrase in a post and think, that might be a good search phrase, but when I search I get nothing. Not even the post where I saw it.

May be you can try Authentication Passthrough Module from Inventua

What I've been doing is just creating a new record in the UserPortal table, using the original UserID, and the new PortalID.

I know this is probably not the best way of doing this, but is there a fundamental reason why it should not be done?