Adds contacts to a contact group. You need read-write access to the group; otherwise, or if the group does not exist, the response is 404 with the text body Not Found.

Send one contact with email or contactId, or up to 300 in contacts. A contact sent by email is created or matched among your own contacts, its meta is merged into the existing meta, and its group list is replaced by this group, so it leaves any other group it was in. A contact sent by contactId is added to this group and keeps its other groups.

Contacts are added in order and processing stops at the first error, so earlier contacts stay added.

One contact returns that contact. A contacts array returns an empty object.

This operation is deprecated. It has no replacement in the public API, and no removal date has been announced.