it really would. my social guild uses a mobile app called Band to handle things like this. it has some cool features like converting automatically to your timezone and letting you chat for build advice while on break at work, but yeah it's not an in-game solution. i'm newly organizing events on 2 guilds; one of them wants to stay away from Band to reinforce how cool and casual we are and the other is just too new-and-growing to expect that level of commitment. it is certainly a thing to ask people to modify their mobile devices just for one out of the 5 groups you're a part of in some videogame. plus then someone has to keep an eye on the Band... i understand the downsides.

apart from making it compatible with player interactions like permissions, and then working within the UI, it could be a very lightweight and easy-to-write function. it's also easy to imagine expanding on a simple one, allowing things like different priorities of events (like if a newer guildie wanted to assemble a fishing run a few hours before the weekly Trial) and something like customized reminders (2 hours or 30 minutes ahead of time, choice of mail or a line in chat) but troubleshooting is tough in this game so the simpler the interface in the beginning, the better.

anyway, i love this idea. i want to support your effort, and encourage more brainstorming. the better we understand our pitch the more likely it is to appeal to the devs.