On the Cisco VPN 3000 Client, you would configure the vpngroup name and group password to match that which you configured on the PIX Firewall.
When the Cisco VPN 3000 Client initiates ISAKMP with the PIX Firewall, the VPN group name and pre-shared key are sent to the PIX Firewall. The PIX Firewall then uses the group name to look up the configured client policy attributes for the given Cisco VPN 3000 Client and downloads the matching policy attributes to the client during the IKE negotiation.