First week: put momma and baby out together by themselves for at least a day or two, so baby learns about "outside" and learns to follow momma's orders when they are out there. (Then in general, if all is going well and there are other mares with foals, you can add them to the herd).
Then after a few days, if you want you can add another mare or two, but watch carefully, as baby may startle with the new arrivals, and we don't want to scare baby into running into a fence or post trying to get away.
Best advice is "go slow" and "watch" how everyone does. Usually, all is fine -- especially if baby has learned to follow the instructions momma gives.
SOOOOOOOOO pretty~!