If you still are looking for an Event sign-up calendar i can suggest you PhpRaider
. Its basically designed for World Of Warcraft and Raiding. Code is fully customizable and you can remove most of the World of Warcraft Attributes to make it just a simple Event Sign-up Calendar. It has also SMF-Login if you install the database in your SMF database.
Tho there are few downsides with that, the ability to sign up according to you membergroup wont exist, but if you are having only one membergroup that will sign you can make a new HTML article and make the calendar show up with Iframe. Then disable access to the article for specific membergroups.
If you are using the SMF-Login as soon as someone loads the page he/she gets into the member list, after that in order to sign up for the events you have to create a Character. You can make through that Groups that the characters belong to and disable sign ups for them .
I dont remember any other downside it might have according to your first post due to the fact that phpRaider is designed for a game.
I suggest go with a demo installation and see all the options it offers. I believe with basic html and php you can even make it pull the membergroups from the database. I wont be amazed if there's such a topic for the membergroup pulling over to phpRaider forums aswell.
Im using phpRaider for my event sign ups for the last 2 years and i don't think im changing it soon.