By choosing the "Members" link located near the top of your group page, you can find the Yahoo! ID and email address of the other players in your group, assuming they have chosen to make their contact information public.
Once you've done that, you can send the player email or use his/her Yahoo! ID to communicate via Yahoo! Messenger.
If you'd like to talk to a player who does not list contact info, try posting a message on the group message boards. This is also a great place to discuss game issues with the other members of your group.