Emails sent to a Group do not get replied to the Group - please advise!

Emails sent to a Group do not get replied to the Group - please advise!

Hello - and please advise!

I have a Group email account set up with two members.  Both members can send and receive emails - the problem is one member doesn't know when the other responds to an email. 

Can you please advise?

I need for both members in the Group to know when an email has been responded to. 

Thank you!