In all likelihood, they are responding on one of their member's behalf. Some members do allow their Group Manager to answer correspondence on their behalf if they are unable to do so themselves.
You should check which Group they are the manager of and then see your Sent Items folder in the Emails section to see if you had sent any messages to one of their members. You may also want to check the Bookings you have made.
If you are being contacted by a Group Manager, unsolicited, regarding their services and/or in an attempt to get you to join their group, you should bring this to our attention by using the 'Report Spam' button.
As per our User Agreement:
We require that you do not abusively advertise or promote third party or your own products or services including by way of the distribution of 'spam' mail.