To send a message to multiple employees or Positions you can CTRL click on each one in the “To” section of your message.
You could create a “position” for that group (ex Part Time) and then set the appropriate employees to that position. This would allow you to choose that position the message to field. (You may want to manually sort that position to the bottom of the positions sort to get it out of the way for adding shifts etc.)
Note you also can create a “Position Group” and then choose that group in the “To” field.
create a group in Messaging that includes multiple positions