how do i add a device to my account

Connect it to your iTunes library and sync it just as you would any other iPod, iPad, or iPhone. There is no limit to the number of devices you can connect to one iTunes library, computer, or iTunes account.

B-rock

But what if the new device belongs to my son? I don't want to sync everything because then our contacts and calendars and apps get all mixed up and synced and that is not good. We just want to share music. Is it possible to add the device but keep all content on each device individually (ie. my son keeps all of his apps, calendar info, contacts etc on his phone but can share/sync my music only). Not sure if I am making sense here.
Thankyou!

Yes. How you sync one device does not affect the other? Perhaps this article will be of a lot of assistance.

How to use multiple iPods, iPads, or iPhones with one computer

B-rock

how do i add a device ( iphone ) to my itunes account ? I dont see any devices listed on the left hand side bar in my itunes account. thx