MyBB Community Forums

If I have a website that already has some users but I want to add a forum to it with mybb can I use my sites user database to create accounts or can I some in some easy way have them use their user accounts on my website to signup for my sites forum I want to build with my bb?

You would need to build a bridge between user data from your site into the forum software you choose.

If for example you use Wordpress for your main site you could use this plugin: https://wordpress.org/plugins/mybbsync/