The best way to use the tool is to set up data alerts against each line so you can keep track of how much everyone is using. I would suggest that you set alerts based off what you think each person will need. If you're not sure, just set up an alert at a set amount (i.e. Let's say you have two people on your 10GB plan, you could set an alert for yourself and the other at 4GB each) and monitor to see how much data each person uses.
If you notice that someone is using more data than the other, you can adjust alerts accordingly. If you're getting close to running out of data, you can easily add more data via the "add data" buttons or you can temporarily pause a line's access to data (until the end of the bill cycle; it will automatically turn on when the new cycle starts).