Can someone explain to a numbskull like me how secure Google Spreadsheets is? As I understand it, it works like this: You make a spreadsheet and send out invites for peeps to access that sheet – is that via a simple clickable URL contained within the invite? So can that authorised person then pass on that URL to someone not authorised to access that sheet, via the same URL link?

I’m looking for a secure sheet, whereby only those authorised can view that sheet – no one else.

I'm pretty sure Google Spreadsheets will work for you. Sending an invitation actually associates that new user with the spreadsheet i think, therefore random people still cannot use it.

The user will need to sign into Google (Google Account) in order to see it, so it's pretty secure, unlike PicasaWeb.