Design and create a simple website, probably only three or four pages.
Users should be able to signup to the website for and pay for an account, Paypal is probably easiest. There should be a way of new users entering a discount code when signing up that reduces the fee.
Users will be able to enter data (numbers) daily which will be recorded. Simple graphs will be created from the data and users should be able to share and compare data. There should be a way of users creating groups with other users for comparing data.
The website should include a forum for paid members, I was thinking phpBB but am open to suggestions.
Text content on webpages should be easy for webmaster to change.