What are the workflows people follow most often when planning things to do?
Organizing activities with friends can be a more difficult process than it seems and I feel that it depends on if you are trying to share this event just with your close friends or if you're just trying to invite anyone who is willing to come.
While I hate to admit it because I dislike the site, since most people use it, creating an event, whether public or private, on Facebook is a good way to get the word out. It's not my first choice, though.
If your friends are actively willing to hang out on a regular basis and want to know about it, I would recommend using something like GroupMe on your cell phone. You can create a group and every time you post a message to it, it will send to everyone, whether via the app or by text.