Make a dedicated partition on it, a filesystem too ;), and mount it to /home/that_users_name
or
just create one partition
and one dedicated directory on it
and mount that directory to /home/that_users_name
That way, the entire drive capacity is accessible to it
but you can create other directories on that drive and mount them to other places
using the drives capacity for those as well.
This way you keep all your users where they belong:
under /home/$username