A couple of days ago, I was in a party with two other people. I was the party leader, and as such, completely locked the party. While we were in a danger zone, someone managed to join us. I never sent them an invite (I don't even know who they were!). They weren't anyone on my friends list, and they weren't on the friends lists of the other two people in the party. When asked, the random joiner said that they just joined through the gate and that they were placed in our party.
I find this a bit bothersome, as it defeats the purpose of having a locked party. I'm not sure what could have been the cause of this, either, unless it has something to do with being in a danger room.
Though, I will point out, that there is a way around having a locked party that is confirmed. I was duo-ing with Deslare, and I was again the party leader. Locked party, same as usual. But his friend (not on my friends list) managed to join the party through joining him. This too seems to be defeating the purpose of having a locked party, especially if it's supposedly locked to friends too.
Like, if I don't even want my friends joining, why would I want someone else's friends to join?
I did a couple of searches, only to get greeted with threads such as "Lower the cost of energy!" and "Let's keep the energy at 3k!", so I don't know if this has been addressed previously.
I tried to track down the specifics of when it happens and when it doesn't happen, but didn't have any luck.
I created a party as locked, and multiple people on my friend's list were able to join. The option to join remained until depth 3, where it disappeared and no on was able to join without an invitation.
When we got to depth 4 (the terminal), friends were once again able to join.
I spoke with another friend who had a party I couldn't join, and he was the party leader and had the same settings as I did (lock party, no checks on allow friends or guild)
In conclusion, I don't know. I submitted a bug, and will continue to do so when it happens and hopefully the data they need will show up.