When you receive an invitation (or after you add in a group code), it will drop into the Invitations tab inside your 'Inbox.' You can find this tab on the left hand side of the screen—under the 'Inbox' and Drafts tabs.
NOTE: This is for invitations you have received. To send invitations, use the envelope icons in your Left Navigation or the 'My Network' section.
If you are joining a group that involves one or more of your children, the invitation will ask you to select the child associated with the class. If you don't see your child listed, 'Create' your child by clicking the pink 'Add New' button and fill in their first and last name. Once you click 'Accept' on the invitation, your account will populate with any of the posts and events that have been shared into the group you've just joined.