Do I need to warn people of my dogs ringworm?

One of my dogs (and myself) have got ringworm (both of us are clearing up and getting better). I have another 2 dogs and a cat who I thought were lucky and didn't catch it. I was at my granddads house (who has a dog) with my little dog and everyone was petting him up and the kids were giving him kisses. I got home today and while making him give high five for a treat, I noticed a bald round patch on his mouth which I think could be ringworm. I don't talk to most of the people who were at my granddads house but are they all likely to get it? Do I need to call them all up and warn them about it or just leave it and see what happens?