Nothing can be done about server activity. If players don't want to fight in arena in that moment, we can't force them. Try it later.

Just ask your Group members to join ;>

(whispers) Just ask your lower-level group members to join, heh~

I've learned not to put off arena fights until the end of the day. If there's an event like a convoy, team instance, or Kyuubi, I go to the arena right after it ends and a lot of people are on, usually.