on, especially when the group is large and the amount of ambient light is low. you should always plan for these kinds of shots and this is another item that you should ask from the event organizer beforehand.
i prefer to shoot groups outside, because i do not have to worry about setting up the lights and making sure that the light is distributed evenly. if you are taking a picture of a group outside, then your biggest issue is going to be putting the group together and making them all look good for your pictures. talk to your group, come up with a nice joke or two to make the 