Sharing members betweeen forums

[محروم.كوم]

I currently have MyBB but will be upgrading to vBulletin in the near future. I want to know if you have lets say 10 different sites (won't have that many but you get the idea) will you be able to share the users? Like if you register on one forum it will automatically work on the other forum as well? I use godaddy so I have the url to the database and all. __DEFINE_LIKE_SHARE__