Ask at peak delight, not at signup. What has the user just accomplished?

Cost it out: the incentive must be well under what you'd pay for that customer through paid channels.

Two-sided programs usually outperform one-sided — the invite should feel like a gift, not an ad.

Reward on activation, not signup — or you'll pay for junk accounts.

Estimate: % who share × invites per sharer × invite conversion rate. Be honest — most products' viral coefficient is far below 1.