On PlaySport, you can organise your staff into groups in the Groups tab. Once you create a group, you can invite people to the group with the Invite button.
People who have been added to a group will receive an email invite to join the group. Note: They might be presented to a 404 error page but there will be a notification saying that they have accepted the invite on the same page.
You can also then message your groups by jumping straight into the Messages tab at the top and sending them a direct group message.