This document provides an overview of Google Groups limitations and restrictions.
As a security measure, the Google Groups for Business service has limits for message activity, group size, invitations, and membership. If you reach one of the limits, your activity is temporarily restricted. The restriction typically lasts an hour but can last as long as 24 hours.
These restrictions may apply to a single group, sender, member, or to all UW-Madison G Suite Google Groups.
Review all policies and limits here: https://support.google.com/a/answer/6099642 .