Sometime last week, just realized the easiest way of persuading friend to add Web Activities to Windows Live. Might not be the most convenient way, but if you know the person in person, it’ll be easier.
Borrow their account and set it up for them (on the same computer). Of course this means a lot of trust is needed, and some setup like Facebook (and Twitter if MS finally implement oAuth to read private timeline) will still need them to login again. But it’s a lot easier, especially for friends that doesn’t adapt quick to new products and able to find their way (and willing) to do so.
Another way might be to use Remote Assistance tool in Live Messenger, but you’ll need a reliable internet connection and the other side would most likely be paranoid about someone controlling their computer.
So I think the best way is for Windows Live to provide a one-click solution to invite friends to use it. Had to create the content (value) first before things roll out.