Players invited to a party receive experience before accepting the invite

[alborrajo]

Describe the bug
A player can receive experience before accepting the party invite.

To Reproduce
Steps to reproduce the behavior:

1. Player 1 invites Player 2
2. Player 1 kills an enemy
3. Player 2 receives exp, even if they're not in the party

Expected behavior
Player 2 shouldn't receive experience from enemies killed by other party members until the invite is accepted.
Similarly, Player 1's party members shouldn't receive experience from enemies killed by Player 2 until Player 2 accepts the invite.