Please help me decide what ZoHo services I need! (newb alert!)

Hi all,
I have checked the forum but can't seem to find my question answered in any one place, so my apologies if this has been addressed already.

I am a volunteer coordinator for a non-profit dance, and as such I need to keep track of what shifts various people commit to for each dance. I have been keeping track of this on a website that only I can edit, and this is somewhat tedious as each change requires me to edit the website and upload the new version. I also have to change many fields by hand that I'd rather have happen automatically using formulas (for example, when someone commits to a shift, the number of needed shifts goes down by one, etc.). Here's a link to my website so you can see what I need to keep track of:
http://home.earthlink.net/~crwpup/frolic/summer09.html

Someone pointed me here and so far I'm not sure how to proceed. Ideally, here's what I want:
*web-based app that allows users to log in and select shifts from a drop-down menu organized by date
*calendar function that shows who's on for what dance
*automated reminders to volunteers a week before their dance
*ability for me to change any and all shifts as people trade, cancel, etc.

Any ideas would be greatly appreciated. Thanks!