You need to build trust in your vetting process, which means you actually need a good vetting process. Some ideas would be to have reviews of the potential guest from their facebook friends. Each review would have links to the reviewer's facebook profiles. Have links to the potential guest's own facebook profile, twitter, linkedin, etc. The more background links you provide, the easier it will be for the wedding couple to decide whether to invite the person or not.

I don't know if you will ever get to the point of convincing a couple to invite a stranger to their wedding, but those things will at least get you closer to that possibility.

If you'd like to brainstorm other ideas on the topic, let me know,

best,

Lee