You bring up some interesting thoughts.
Quest giver: Oh, that sure is a lot of you that showed up. You there, PC name who has some sort of correlation with this event(Or simply the first PC to show up if there isn't anything to correlate to), pick X number of these people to join you on this event.
I think turning people away from events should be a hard no. Let's face it, lots of us have a lot of things going on in real life. A lot of the recurring PCs play mainly on the weekends. I have seen a few at least. What if there is an event scheduled, they log on for that event just to get turned away? That would be extremely disheartening to the PCs involved. I could see this idea working if there were multiple DMs doing different events in different areas of the server, but usually, if an event is going on, all of the DMs are focusing on that one event.
If the DM or player wants a smaller group of people to attend, it should be made clear ahead of time and usually without telling the whole server.
Secondly, let's talk about RP and IC reasoning. Some of you might do this, some of you might not, some of you might have different opinions. Leave your ideas below! When DMs make announcements for events, they usually do a good job at letting us know what kind of event it's going to be exactly, or what sort of atmosphere we can expect. Earlier today there was an event that basically came off as some shady person was looking for people to do some shady business in a shady part of the world.(Upperdark) Did it sound fun? Heck yeah it did, but I didn't go. Why? Because I believe that no good aligned character would have gone, let alone a paladin.
I agree with this to a degree. A while ago there was a pair of events. The first one was helping Sullivans capture slaves and the second one was to help reclaim the slaves. I had originally planned on attending the first one but got caught up in something. After the event, I heard it was an event about capturing slaves. I, as you do, didn't think my character would be interested in that and most likely would have turned it down after we found out what we were doing.
All though, as I am thinking about it now the event still could have been interesting for my character. If your character morally doesn't agree with the mission, they could do something that causes the mission to fail. Using my event, if I had gone on my character I could have purposely got the group caught or PVP'd and attacked the party.
And lastly we have level ranges/power levels! So let's say your character is online when an event summon is made, awesome! You hurry over and you're one of the first there, great! You even have an IC reason for being on this specific event, fantastic! Buuuut you still shouldn't go. Why?! Simple, you're just too dang strong. Yes, we all like being strong, and some of us like being the hero or center of attention more than others, but being that guy can sometimes be a negative thing.
This one falls to the DMs a bit. If your character is level 15, but the rest of the group is lower then the DM should recognize that and balance the encounter accordingly.
Empress of Neon has an interesting suggestion where the DMs give a different role to the lower levels.
Level 15+ character only get XP from DM events. Now, yes you shouldn't be coming to events just for the XP, but if you have a reason to be in the event why not. Plus the people playing those character want to be involved just like the lower levels. They are obviously enjoying that character so they should attend any and all events possible.
I get what you are saying, but aside from the DM specifically level capping an event, I have to go back to what I said at the start. No one should be turned away from events.