2 blogs one members database?

i currently have three blogs, only two of which i am trying to use the same registered members for the two blogs. one is a main blog with tech news the other is a forum. at least im trying to make it a forum. but in order for me to do that, i need to be able to use the same members that registered on my main site so i can have them log in on the forum. they are under two different domain endings, .us and .info. the blog is .us and the forum will be a subdomain of the .info. is there a way i can use one database of registered users for two sites? if yes, how? and if there is a plugin, which one? if not, would someone be able to make one for me? i am not that good with php programming and wp programming.

thanks alot.