Private Forum for customers

[pawan]

I want to create a private forum for customers only.

What I'm doing currently is that I have set password for that particular forum and give the password to the customers when they buy products from me.

The problem is they have to enter password every time they want to visit the forum.

Other alternative I am thinking of is creating a separate "Customer" usergroup and give access to that group only. This is the perfect solution for me but how to automate it so that I can give customers the instructions for automatically upgrading themselves to "customer" usergroup? I don't want to manually add all the new customers to the usergroup.