I take it you want it to be automatic rather than have the user re-register on the child portals (if the username/password, email, etc are the same the same user can be on multiple portals without a problem).

I don't know if there's a module out that does this (I think there is but I've never had a need for it myself) but if you know SQL or know someone it would be fairly easy to setup a stored procedure or trigger to do it for you. If you look at the database you'll see that there's a UserPortals table. If you have to UserID and the PortalID you could update that table and your user would be able to log into it without having to register or need new info.

Since the search in these forums never work, I went through posts manually until I found the answer to our common question.

Here is a module that will allow users access to multiple child portals by logging in only once.

http://www.dnnmasters.com/CB/tabid/99/CBModuleId/407/ArticleID/3/DNNMasters-Multi-Portal-User-Sharing-Extreme.aspx