I have recently made a new forum and whenever i try to let my friends register, they will got to type in there username and other information and when they try to register it says there username is already in use, and there in nobody registered other than myself.. Can anyone assist me please?

Check here:
Administration Panel > Users & Groups > Users > Inactive Users

I checked under inactive users, there is no names listed there, and they are having the same issues, Such as a friend tries to register with a username "Josh" and it says "This username is already taken" I'm not sure what's going on.

Hello,

Try this:
Admin Panel > General > Security:
Unauthorize subscriptions made with the most commonly used spam usernames. : Set it to Yes > Save

See if it will fix the problem, please take note that setting this option will make it an open gateway for spam usernames.

Thank you that fixed it!