AFAIK, for Google Groups, there is no limit regarding the number of members. There are limits regarding about how and when they are subscribed to the group.
Google Groups documentation
Bear in mind that there are two main "brands", Google Groups and Google Groups for Business.
Google Groups (
Use googlegroups.com as the domain for the email address of the group.
The official documentation for end-users is in http://support.google.com/groups.
The official documentation for accessing Google Groups through scripts is in https://developers.google.com/apps-script/reference/groups/.
AFAIK there isn't an official documentation regarding
Google Groups for Business (
Use a custom domain for the email address of the group. It has the same features as
GG plus some other features.
The Groups administrator FAQ estates a limit of 100 members for groups of "Google Apps". In that help article, by "Google Apps" they are referring to the Google Apps for your domain, free, legacy, edition. The Google Apps for Work or Education editions doesn't have a limit on the number of members for each group.
From Groups Policies and Limits - Google Apps Administrator Help.
As a security measure, all Google Apps groups have limits on message
activity, invitations and size, and membership. If you reach one of
the limits, you are prevented from the activity whose limit you
reached. The restriction typically lasts an hour, but can last as long
as 24 hours.
The following limits apply for Google Apps for Work or Education
accounts. The value of these limits may change without notice in order
to protect Google's infrastructure.
Go to the above link to learn about the detailed group limits.
Documentation related to