It appears that only one person per IP address is able to PPTP into the RF660, is this correct? Can it be increased? Do the same rules apply for an IPSec connection? I'm talking about an individual ipSec, not the IPSec between other Multitech's.

That is correct! You can't access the router from multiple hosts on the same network with any client, PPTP or IPSec. You must create a VPN tunnel between two routers to allow two or more Hosts access from the same network.