What we used to do was come up with spontaneous events that involved other players. At times, we even had events where only one or two SeeDs attended, but we gathered a group of players who we found in the game at that moment. I'm not sure if others did this, but sometimes I even created events that were so spontaneous that they didn't even get mention on the site until
after they happened. If one feels ambitious, a single player can create an event that entirely consists of random people.
It's actually pretty fun, because you can sometimes make quite a scene, and you know that it's entirely your fault.

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I understand entirely if most (or all) of us don't feel up to doing that, but take my word that it can be pretty fun.
What is more realistic is for a small group of us to effectively accomplish the same thing as the above. The trick is coming up with something unique that would either benefit those who become involved, or just seem unusual enough that people would come just to see it happen. I wouldn't recommend the pointless insanity that we sometimes enjoy, but events where players will realistically want to spontaneously join.
In Runescape, clan chat is a must. We could have gone way beyond with our old events if only that feature existed back then. Before starting a train, or lighting fires, or doing anything else of that sort to get attention, try to get players to join our channel. Not only will it make things easier to organize, but there may be an element where those players also invite their friends. It also looks
good for our guild because the first impression will be one of activity. Lastly, they'll probably stick around for a longer time during the event... Or if they leave, perhaps they'll return!
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The following are some tips that may help...
- In Runescape, trains and fires can get attention. Just be sure that something more "profitable" or "worthwhile" follows whatever attention grabber is used. Most
random players will leave an event if this is all we end up doing.
- Parties in semi-active places are great. This applies to pretty much any MMORPG with social aspects.
- A sense of belonging is important. You may want to get creative, but another example in Runescape would be handing out blue capes for those who want to join the event. Anything that makes everyone seem less "random" is good.
- As another tip for SeeD, drop parties can be
very effective. For a few years, we had more drop parties than any other kind of event.

Of course, I'm not sure if it's possible anymore due to the recent game updates, so maybe an active RS player can correct me.
- In Guild Wars, the best public events can most easily happen in semi-active districts. There should be many players, but not so many that we can't dominate the district. It should also be an important location so the players aren't quickly running in and out all the time. Since most new characters are alts, perhaps we don't have a particular advantage in newbie areas in comparison to any others.
- In many MMOG's, firework abuse can get attention.
- Try to make friends with those who come. For example, don't
just use the guild channel. Private messages can be very good to send, even if you're merely saying "Welcome!".
- Have fun with the event! If
you're not enjoying it, don't expect that others are. If this is the case, change things around so the event is better. There's nothing wrong with switching the purpose as long as everyone currently at the event can still be involved.
- It's okay for events to "fail". Events won't all succeed, and chances are that your first ones won't be as successful as you wanted. After a few attempts you'll figure out what works, and after that it will keep getting easier! Besides, if others are also trying things, our guild should be growing. As we grow, events will naturally grow as well.