I read this article, which outlines some limits of Google groups for apps: http://support.google.com/a/bin/answer.py?hl=en&answer=167085
For instance, only 100 members are allowed in a group.
However, where can I find limits for generic Google groups (https://groups.google.com/)? What is the maximum group size? How many users can join in a single day? What are attachment limits?