In order to send account activation invites towards multiple users/families, first you need to access the "Families Management" section from the menu.

Then click on "Active children" and filter the list by "Unregistered users". Then, next to "show" select 30 so that it will show 30 families in the page, because when you send invites it will send them to the users in the current page. Then click on the empty box next to the "Family" column to select all families in the current view and you will see the "Invite users" button.