Ever have to reform your party to fix the formation and this random person keeps forcing his way in and will not take no for an answer, but locking the party isn't an option since you need to reform? This happens to me daily. Listen, I'm sorry, but an Engineer, White Mage, and Paladin party doesn't need a ninja trying to take the front row away from the engineer, for example. And that exact example has happened to me four times. On my level 32 engineer. Who hasn't even set foot inside Magnes cave yet.
I propose a party invite option, where you can invite people next to you to join you while maintaining the lock. This way, the party leader can easily organze the formation while keeping rows in consideration, as well as only inviting the people they want. This is useful because enemies hit harder with more party members, and sometimes it's better to streamline a 3 person party than to load up on a full 5.
If I sound like an anti-party guy, I'd like to say I'm quite the opposite. Solo is boring. However, some parties simply do not work. Ever tried Zeromus in a Party with 3 mature callers, a mature black twin, and a back row paladin? I did. That was brutal.