For 5 users (dynamic IP-ANY), how many VPN entries do I make in the RF660 for all 5 PCs to VPN in at the same time? Can all 5 PCs use the same secret key (so there is only one entry in the 660 with that key), or do I set up 5 different secret keys?

Since all the PCs are using dynamic IPs, you only need one VPN connection setup in the RF660VPN, all five remote PCs will be able to connect to the same VPN connection that is setup in the RF660VPN.