Contact groups can not only group individual contacts, but also can include companies.
Adding a company to a contact group is simple and easy.
To do so, navigate to Companies → Select a company → Assign Contact Groups.
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If you click on the “Assign Contact Groups” button then you will get this popup window to assign groups.
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Check the group that you want to assign to a company and hit the ‘Update‘ button. This will add ONLY the main email address associated with the company, not each contact.