A measure of total success for an advanced system is whether it allows group members to be out of sight for long periods, but to link up again as soon as they wish. However in public places such cords quickly interfere with other people there. Or a strobe they wear round their heads gives off a huge flash directed up to the ceiling, which you can see from afar, without their being dazzled. Similarly, on a button press, chains of illuminated arrows on the wall guide the inquirer to the next member. The timing might be enough to distinguish between groups i.e. only the arrows or beeps that go off at the moment I press my button are relevant to me.
Active badges inform a computer where everyone is.
I.e. consider a wrist device, or pocket device. Also, shows the technology should be able to deal with tens of thousands of people at once. Initial field studies could ask organisers of each target site how many and what kind of problems occur of people losing other group members. But I suspect that this is an example of something that most people manage OK, but would adopt something that made it much more effortless. E.g. built into audio wands in museums, into PDA museum guides, etc. etc. and offered as an add-on service.