The idea is that when you have a pack or den activity, a leader creates the activity for the entire pack or den, and then parents can join their Scout(s) to the activity or a leader can add everybody’s information. If Scouts are doing service projects on their own (not part of a pack or den activity), then the parents (or the Scouts for older programs) can add their own information.
As @Stephen_Hornak said, the service hour details are now being fed to the BSA’s service hours tracking system, so there is no need to separately enter them for JTE at the servicehours.scouting.org web site. (Exception: Eagle Scout service project hours are usually entered by districts / councils as part of the Eagle Scout application process.)
When you add a service project entry, there is a place at the bottom where you can include hours for non-registered youth and non-registered adults.