I would like to know if anyone has ever heard of including an enclosure in the invitation that says Ribbon Guests for family to give the ushers when entering the church.

I've not heard of it and can't find anything googling.

What would be the intention of doing it? Would it be for a seating issue?

I have not heard of this either, I do have and reserve certain seats for certain familiy members, but these are normally marked with bows and ribbom on the pews.

Yes this is for seating. Instead of the ushers asking whether you are family, the guest gives this card to the usher. I had never heard of this until my friend wanted to enclose these in her daughter's wedding invitation.

I see. I certainly don't see any harm in doing it that way, but am not seeing a benefit either, since the ushers will be speaking to each guest anyway... but if the idea is appealing to you, go for it ;)