Try this: Open Contacts.app, select the group name on the side bar, and if you want to share the full list, select all. Right click on the selection and select “Export vCard…” and save it. You can then send that file to your friend. E-mail should probably be the easiest method.
If you want to send a sub-group from the list, you can individually select items (Command-click) and then save as before.