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