If you see a Join this Group button in the upper right of a group's details page, you can send the group owner a request to join the group. As a member, you can access all the items shared to the group, which may include some items not shared with the organization or the public. In addition, you may be able to share your maps and apps with the group if the group owner allows member contributions. Check the group details to see.
To join a group, verify that you are signed in and open the details page for the group you are interested in joining. Click Join This Group and click Submit Request. The owner of the group sees your request on the group's details page and accepts or denies your membership. If the owner accepts your request, you are automatically added as a member, and the group appears on your Groups page.
You receive messages in the Notifications pop-up when you request to join a group, when your request has been accepted or declined, and when you've been invited to join a group. The number of notifications you have appears next to your name at the top of the site. Click the number to view your group request messages.