Now that the group is defined, we can start marking shots. Notice the new button:
As usual: click the button, click on target where the shot is, confirm with "ok".
(marked the shot nearest to PoA)
As soon as you mark a second shot, two additional elements appear on the image:
The blue line is extreme spread, connecting two furthest shots in the group. The orange circle is the average point of impact (PoI). Both are calculated immediately as you add or remove shots. Here is the final picture of the group:
The "Delete shot" button deletes the last shot marked in the group
If you want numbers, just toggle the "Display groups infoboxes" checkbox.
A box appears with group's extreme spread in your favourite units, and horizontal+vertical distances between the group's PoA and average PoI. (I did not bother to calculate or display any other "statistics" for individual groups, see "Why TARAN does not calculate and display more information about groups?" FAQ item.)