MyBB Community Forums

Hello,

I don't want to create another usergroup and then allow them to access a specific forum, I just want to pick some members from the memberlist and allow them to access that specific forum, is that even possible?

Try this plugin:
http://mods.mybb.com/view/custom-user-permissions

That plugin looks like a good solution BUT I still wouldn't recommend doing it this way.
You will eventually lose overview of who is allowed where and which board is accessible by whom. I would create a group just for that specific subforum, name it e.g. "subforum1-users" and then add this group as a secondary group to the required users. That way it's much easier to control and it doesn't interfere with your usertitles or whatever.

...or just set a password for that specific forum.