The MassMessage senders group was created to allow non-administrators to send global "mass messages" from Meta.
This usergroup allows users to send messages to all WMF wikis using the MassMessage interface. The rights assigned to this group are autopatrol (Have one's own edits automatically marked as patrolled) and massmessage (Send a message to multiple users at once), which are also held by administrators and WMF staff who are in the staff global group.
As the successor of the protected Global message delivery/Access list, this status can be given and removed by administrators. Requests for normal users can be made on WM:RFH, while Wikimedia Foundation staff should instead request this right from Wikimedia Foundation, see WMF Staff userrights policy (restricted link).