If the team exists in the Teams pane, you can make a preassignment in either an MSP installation or in the Admin view of the Web Terminal for one member and this will automatically preassign the entire team with them.
If you want to assign a group entirely through the Preassignments pane, navigate to the pane in either an MSP installation or in the Admin view of the Web Terminal and click the "New Preassignment..." button in the upper right corner (in the Admin view of the Web Terminal, click the "+" button). 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.