One way for students to be a part of more than one teacher's roster is for all teachers to use Sign in with Google and import the same classroom. 

Another way is using Colleagues from Classes to send and receive invitations to share students. Premium accounts can invite colleagues to share. Free accounts can accept invitations, but cannot issue invitations.

CAUTION: If you use this feature, you may need to upgrade your account to accommodate more students.

Go to Classes and Invite a colleague using the new button that appears at the bottom of your class list. 

You will need their Boom Learning account email address to invite them.

Once they accept you will be able to share students. 

You will see a badge if you have been invited to share students.

Click requests to respond to invitations..

Then click a Classroom and click Share Students with Colleague.

Select the students to share.

Confirm and click Share.

The recipient will receive a notification of the share.

Accept or reject the share.

A new classroom will be imported with the created students. You can use move to move the imported students to a different classroom.