Re: Confusion about sharing databases
You only need to share databases to have integrated user logins. If you just installed WordPress, it’s probably 2.6, which is not compatible with bbPress at the moment, so you do not want to attempt this right now anyway.
The difference is, when you share a database, is that bbPress will use the wp_user and wp_user_meta tables for it’s users, rather than creating bb_user and bb_user_meta tables.
If you don’t care about logging in in bbPress and being logged in in WordPress, and vice versa, then you don’t need to shared the tables.
If you don’t do an integrated installation (sharing of the wp_users and wp_user_meta tables), then you can just use a different table prefix (by default WordPress uses wp_ and bbPress uses bb_) in the same database and not worry about a thing.
This is the best documentation I know of:
https://bbpress.org/documentation/integration-with-wordpress/