I think the PC's need individual public addresses.You can do this with the following example to set up a pool.
!This pool gives you 40 translated addresses, but I think they must be public
global (outside) 2 xxx.xxx.xxx.1-xxx.xxx.xxx.40 netmask 255.255.255.0
!
!This line is the private network of the work stations
nat (inside) 2 192.168.2.0 255.255.255.0
You may have to open up udp 500 and protocols 50 & 51