I have a problem. Whenever someone tries registering for my forums it says that the name has been taken. I check my member list and it says there is no one named that username they are trying to use. Help me please.

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

There aren't any users listed.

Ok. Try this:
Administration Panel > General > Forum > Security
Unauthorize subscriptions made with the most commonly used spam usernames.: No
With this off, spammers/spambots will be able to register.

I got it to work. When I set the New accounts activation to "User" it doesn't let them register for some reason. I switched it to Automatic and it worked like a charm.

Activate by user it that a user have to check their email for your forum mail to activate it. Maybe they didn't activate it and then they try to register with the same username, I think that's why the forum said "the name has been taken". Search in your inactivate users I used to have the same problem

It's not that they couldn't get the email to verify. It would say the username is taken. Once after I switched it to automatic it worked.

The user above explained why it said that.