I am trying to play some packs with a friend and it wont let me invite him to my party. Also, the overlay keeps randomly popping up and getting in the way, getting real frustrating if you can imagine.

What versions of BetterQuesting are you using? Also the book expansion is 3rd party and not maintained by me.

Quest book opening randomly happens when someone else in the world opens up the BetterQuesting GUI.

I don't know about the other user but I am having almost the exact same issue, along with the other player being stuck in edit mode. Using BQ version 2.3.179 and Better Questing Triggerer 0.4 in the Forever Stranded modpack on curse.

Edit/Hardcore mode syncing was fixed as of 2.0.185 and party issues were fixed as of 2.0.194