first you need to identify the diseases that you may contract. realize that some diseases are not a worry to an older goats.
the third is to maintain good hygiene during the gatherings. dont share water buckets or corrals ect
For organized groups, if you are really concerned, you can ask that folks periodically have their vet complete a certificate of inspection. We have to that for shows/fairs anyway, so we usually have one that is fairly recent. Pretty good idea for interstate travel to do this as well ... which would probably mean it should be done for a lot of folks traveling to Rendys.
That said, a "self-certification" goes a long way as well ... ask everyone to make sure their goats are healthy and leave them home if they aren't.