Great question — answering as the UphillTech team:
I would not want to ask them to do it once they've signed up: aside from scaring them away, they don't really know much about the service and therefore will not be very effective at promoting it. I would target the people who have been using your service for a decent amount of time, such as one month, as they will be much more experienced with your service and do a better job promoting it.
Full answer by user6901: https://webmasters.stackexchange.com/a/29361 (CC BY-SA 3.0).
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