When configuring your contact groups, you have the option to use the contact’s domain and/or specific email addresses from within the customer’s organization.
As your mailboxes are already linked, you would not need to add your own email addresses on the contact groups or contacts screen.
To add a new contact group, browse to Tools > Contact Groups
To add a new individual contact, browse to Tools > Contacts
Once the contact or contact group has been configured, you can immediately select it under the ‘When communicating with’ filter.