How groups work:
Google groups acts as a mass emailing tool, Q&A forums, collabritive inboxes, or Web forums depending on the use case.
When sending an email to a google group, the email will be forwarded to all members of the group. For example: if you need to email all managers at snowbowl, sending an email to managers@snowbowl.ski will forward your message all members currently assigned to Managers@snowbowl.ski google group.
Google has set the following limits to adding members outside of MCP:
External member additions at one time: 10 per session
External member additions per day, per group: 100 per day
External member invitations per day, per group: 500 per day