Can I message a Google Group from the EarthMate App in order to send a message to more than 10 people? I created the group and "own" the group... Any other tricks to messaging a large group?
As long as it has a phone number that can handle text messages, you can send to it. If this is a Google Voice number, you should try it with your own Google Voice number just to be sure. I only mention that because Garmin uses third-party SMS gateways which are (normally) aimed at interfacing with the cell networks.
AFAIK, the only limitation on sending a single message to multiple destinations is the size of the address list. iR messages are 160 characters - the message, the recipient list, and any overhead (such as your position) have to fit. The more recipients you have, the less space for the message text. You can minimize the recipient list size by sending to contacts, rather than to fully specified addresses. This does require you to sync the device EVERY time you make changes to the contact list.