Update: looks like you can invite 16 players, but the actual party chat is still limited to 8. The article below has been amended to reflect this.
Looks like the PS4 firmware update 1.71 just released by Sony Computer Entertainment included an undocumented feature.
The limit for invites to the Party Chat feature has been upped from 8 to 16 players, as you can see from the screenshot below, even if the limit for the chat itself still seems to be locked at 8. This is probably to let players invite more friends, considering that not all of them might join.
It could indicate a future increase of the limit in the actual chat, but for now there’s no way to know when and if that’ll happen.