Bloomz allows you to add a teacher(s) or parent(s) to multiple groups at the same time. Please see the steps below 

• Click on the school in the left menu under Filter by communities
• Click on the Manage icon on the top 
• Click Classes/ Groups tab
• Select the classes by clicking on the circle on the left of a class
• Click Add Members at the bottom 
• Choose Role
• Click copy-paste emails
• Copy the emails in the To field
• Click Send

Invites will be sent to all the emails of the parents,  for all the classes chosen!