MyBB Community Forums

i have a forum and it will have a private unseen group.  when a member signs up I would like to be able to have them enter the code to gain access to the group.  the group is not visible to anyone that is not a member of that group

Set password for the forum.
You can dispatch the password to users on successful registration.

If you are talking about entering code for group and not the forum then use group invitation system. How come someone enter code for a group they can't even see?