As noted in the images below by a credible player, if you stand directly above Kraknaknork's dimension and another player is near Greenshore, it makes it possible to yell from Mainland to Rookgaard. Kraknaknork's dimension is the 1st mission of The Rookie Guard Quest which from my understanding is on Tutorial Island which was before Dawnport was implemented in 2014. New players won't be able to access this island it's only if you're on Tutorial Island that you can complete Mission 1.
Also noted, if 2 players have characters on Tutorial Island it's possible to share XP. Fun information, the player reported a bug as he claims it's also possible to share XP from Mainland (Greenshore) to Rookgaard (Tutorial Island). There are not many creatures in Greenshore but he said for example the Insectoid Invasions which are rare.
Source of information/pictures and special thank you to: Reddit post by DarkGladir: It's possible to share XP from Mainland to Rookgaard