Basically make them locked to a party forcing them to log out of the game before they can accept any new invites.
How to:
1. Invite who you want to join target
2. /kick the guy you just invited, BEFORE he accepts.
3. ???
4. PROFIT!
It does so they appear as in a group, so when other people try to invite him it looks to the people inviting the victim as if they're already in a group
*EDIT*
I figured out you need to be in party with somebody for it to work properly. So basically you can do this while finding people for parties. <- Not sure if it actually locks another party slot, but it's still cool.
Even if they click decline they're still locked to the group until it disbands.
UPDATE!
Still works! Updated:
Requirements:
- Being the leader of a current group/raid, with at least one slot open/free. Inviting him without being in a group doesn't work.
- Victim to trick
How to:
1. Invite victim.
2. /kickbefore (s)he accepts.
3. (S)he's now locked to your group, and can't leave without relogging. I'm quite sure that even /script LeaveGroup() doesn't work.
Not sure how it works if (s)he declined.