Only a user with the "Manager" profile can change the groups to which an Educator/Assistant has access.
To add an Educator or Assistant to multiple Groups simply:
Step 1 - Select the "All" Group
Step 2 - Click on "People"
Step 3 - Select the Educator/Assistant to give access
Step 4 - Choose the "Edit" option
Step 5 - Select the "Add Group" icon and choose the Group(s) you want
Step 6 - Hit the "Submit" button
You can check this article to see how educator access the different classrooms