The computer does not have to be in a domain, but you need the users to exist in your ad so you can assign them to machines.
i have an Querry.
If the Systems is not in Domain, how can we assign the user to that particular system?.
The same way you would do for systems in the domain. Install the agent, sync, and they'll show up in ePO where you can add users. You still need AD for the users!
exactly what georgec said