Hi,
Unfortunately up until AOS 7.1.3.2, the only way to add ports is to re-apply the full list of ports you wanted to add as you noted below.
With respect to minimizing the time figuring out what ports are allocated to a group, you can use "show interface-group-config gigabitethernet <group-name>" or "show running-config | begin <group-name>".
I'm happy to report we enhanced the "apply-to" command in AOS 7.2 so that you can now use operators like "add" and "remove" to easily add ports as needed.
(ArubaS2500-48P) (gigabitethernet "default") #apply-to ?
<interface-list> Replaces the existing list with new list. Enter
valid interface group members in ascending order
i.e. 0/0/1-0/0/5,0/0/10,0/0/21-0/0/25, max 255 chars
add Add interfaces to the current list
remove Remove interfaces from the current list
Best regards,
Madani