Say I have 5 forums. I want that when somebody register in one of the forum, he/she automatically registered to the other forum and he can log-in in 5 forums with the same username and password and its not required to register in all the forums.
Can it be done?
I'm pretty sure you can do this by having all your forums using the same SQL Database. However further details are beyond my knowledge.
I'm sure you could do this by just pointing every install to the same database.
There are tons of threads and discussions on this. It can be done and some people have succeeded the the past. The trick is to point all the 5 forums to the save database that contains the user information. Just search around on MyBBoard and you will find more information about it.