Also, if the network config is like sc302 told, you will have to make some user in AD Users and computers on DC. In the process of joining client to domain you will have to provide login details of domain user you created on DC. Remember, when you join domain, and login on client as domain user, your rights on local computer will be changed unless you define domain user as admin on local machine.
this makes as much sense as the ramblings of a drunk person high on coke...you ok there?
You can join a pc using the domain admin account or any account that has domain admin rights, you do not need to be a user. After the computer has been joined to the domain followed by a reboot, any Active Directory user can logon to that pc. By default Domain Users have the ablitly to logon only and run programs, they are not Administrators or Power Users so they cannot add software or hardware or really make many system changes to the pc. I would suggest making a AD group for local Admins and Power users and adding them in the respective local groups on the PC's so that you can easily elevate their rights through AD vs being on their PC. Or you can just Add them as needed to the PC. It all depends on you really.
You can add a computer as a end user, but you surely do not have to. You can only do this 10 times as an any one particular AD user without any other configuration, unless you are a domain admin then you can add as many computers you want to the domain.