If someone knows they will be unavailable for a period of time, they can tell MindMyMinistry about this, and the system will then automatically warn you if you try and roster someone on for a service during their leave.
While logged in to your MindMyMinistry account, click on your name top-right.
Next, in the Leave section half way down the screen, click on Add Leave:
A form will pop up where you can enter the dates you'll be away, and a comment.
Click Submit, and you're done!