OK, so regarding this, I think I understand what’s happening.
I got an Alliance character boosted from level 10 to 70 today. The booster was a level 70 Horde character. When he invited me, it was as a raid group. Then, after I was summoned to the southern stone of Azure Span, the group was converted into a party. He didn’t use party sync at all, but I could see him, and he could see all the other boostees.
I was trying to figure this out because I had attempted to do it myself—I could have boosted my own character using my other account. However, I couldn’t get Chromie Time to work. My other account had a level 70 Horde character, but since I couldn't use party sync due to being different factions, I tried using a raid group instead. I attempted various things—forming a party, logging out and back in, etc.—but no matter what I did, I couldn’t get my alt into the same phase.
I tried with no Chromie Time selected, then with Dragonflight selected, and tested every possible method, but nothing worked. However, when my Alliance character got boosted, as soon as I reached level 70, I phased away from the entire party—including the level 70 Horde booster. That seemed super suspicious to me.
At this point, I’m assuming it’s an exploit since there doesn’t seem to be any legitimate way to do this, aside from maybe being on the same server and hoping for the best.
But can't do this in a cross faction group. Im invited my horde friend to group then stay party sync so i can see him in map and no more Timewalking campaigns but when invited my alliance friends hes removed from group