Internal DHCP server on Aruba controller is not supported past 512 concurrent leases, so, as long as you are in that scope, you should be fine.
To set this up, create a new VLAN, assign an IP to that VLAN interface that would be the guest clients default gateway. You then need to enable DHCP server on the controller (service dhcp) and then create a scope that matches the subnet you configured on the VLAN interface. Once this is complete, map the Guest Virtual-AP profile to that VLAN ID and you should be good to go. If this is a non-routable subnet, you may want to enable "ip nat inside" to that VLAN interface.
Regards,
Austin