They should not allow level 1s to initiate a party invite. They can be invited to a party, but cannot actually make a party of their own. Or place a level limit until you actually hit your first dungeon (whatever level that may be when you actually unlock Sastasha through MSQ). Then, I guess, lower levels could still group up if they are legit players.

I'm bad at explaining:
  • Level 1 can invite a level 1 to party.
  • Level 1 cannot invite a level 10 to a party; the level 10 can invite the level 1.
  • Level 15 hits Sastasha and can invite any level now.
  • Level 40 cannot invite a level 41 because they never did the MSQ to unlock their first dungeon. The level 40 can invite a level 39 or lower to party.

Basically, add two flags that restrict invites. First flag being lower or equal value of current level to initiate invite. The second flag being Sastasha unlocked (not completed).