hey C Warner,
1) create a new formula named Seconds (to place on the Details section) with syntax like
timevar t := {your time field};
numbervar s:=(hour(t)*3600) + (minute(t)*60) + second(t);
2) right click on the Seconds formula and choose Insert > Summary > Sum and also change the Summary Location to be your group
3) now right click on the Seconds formula again and choose Insert > Summary > Average and change the Summary Location to be your group
4) right click on the Summary and choose Format > go to the Common tab > Display String and enter this syntax which will change the output of the formula from seconds to a readable time display.
numbervar ts:= currentfieldvalue;
numbervar hs;
numbervar ms;
numbervar ss;
hs:= truncate((remainder(ts,86400))/3600);
ms:= truncate((remainder(ts,3600))/60);
ss:= truncate(remainder(ts,60));
stringvar display:= totext(hs,'00') + ":" + totext(ms,'00') + ":" + totext(ss,'00');
display
5) if you want you can also move this summary to the Group Header and then suppress the Group Footer.