Post-wedding celebration etiquette

Hi!

My future in-laws are trying to host a Post-Wedding party for us (we're having a destination wedding) and future MIL asked us to gather phone # and emails from our guest list so she could send an e-vite. I have most of my guests emails but my fiancè still has to gather the ones from his co-workers. He is now trying to go through a loophole by creating a FB private invite to send it to his coworkers and our guests.

I was okay with it till I realized the original invite had our registry information. If we do the FB group invite, we would be sending out the invite & would show up as co-hosts of this party in, which case, I feel is very tacky to add the registry. I say it should come from his mom exclusively due to the fact she wants to add our registry information.

Any suggestions?

Thanks!