If the team exists in the Teams pane, you can make a preassignment for one member of the team in either a VSP installation or in the Admin view the Web Terminal, and this will automatically preassign the entire team along with them.

If you want to assign a group entirely through the Preassignments pane, navigate to this pane in either a VSP installation or in the Admin view of the Web Terminal and click the "New Preassignment..." button in the upper right corner. Hold down the Ctrl key on your keyboard (or the Command key, if using a Mac) to select all members of the team, then click "OK". This will allow you to preassign a group of people together.