Watch Google Groups vs. Google Contacts:
Google groups vs. contact groups:
You might be wondering how Google groups differ from the contact groups you can create in your My Contacts list in Google Contacts (click Contacts in Gmail).
The primary difference is that a Google group has its own email address, so it can be shared in our Groups directory and others can send messages to it.
Your contact groups, on the other hand, are for your personal use, to make it easy to add a list of addresses to your email messages and meeting invitations. They don't have their own email address, so no one else can send messages to them.
Google groups and shared global mail lists contacts: Google groups aren't included in the contacts you access in Google Contacts. Instead, you'll find them all in our Groups directory. However, after you send a message to a group, it appears in your All Contacts list in Google Contacts.
Google groups and "auto-complete": After you send a message to a group, it's automatically added to your All Contacts list in Google Contacts. Now, whenever you want to send another message to the group, invite the group to a meeting, or share a document with the group, just start typing its address and it will be completed automatically.