See also the official documentation:
Help Center > Members Manual > Additional information > Standard group email addresses

To get a help email for a top-level group (i.e. not a sub-group), send an email to GroupName+help@groups.io. Or use the link on the group homepage.

To get a help email for a subgroup, send an email to SubgroupName+help@GroupName.groups.io

You must substitute the name of your group and subgroup for SubgroupName and GroupName.

Here's the automated reply to my email. 

Clicking on the link on the homepage of my group brings up an email that I must just send. No info needed. (I did get a prompt from my email account about the blank subject line - I ignored it.)

The reply contains: group description, posting email address, how to get group guidelines (if it exists), changing your subscription (email commands), how to contact group owner, 

Group Description

And email commands.

Posting To This Group

GROUPNAME+subscribe@groups.io

To join a group

GROUPNAME+unsubscribe@groups.io

To unsubscribe from a group

GROUPNAME+help@groups.io

To receive an email containing a list of these commands

GROUPNAME+digest@groups.io

To receive plain digests instead of individual messages

GROUPNAME+fulldigest@groups.io

To receive full featured digests instead of individual messages

GROUPNAME+summary@groups.io

To receive a daily summary instead of individual messages

GROUPNAME+single@groups.io

To receive individual messages instead of digests or a summary

GROUPNAME+special@groups.io

To only receive special messages

GROUPNAME+nomail@groups.io

To stop receiving messages via email (you may still read messages on the Web)

GROUPNAME+owner@groups.io

To contact the owners of a group

Of course the email has the actual group name!