First I would stop reminding the group they are going outside after snack since it is a routine they all know it anyways. That means the child can eat her snack without worrying (crying) about the next event. Then just dress her first yourself so it is quick and before she can complain. Then get everyone else ready while she screams. Then just go outside. Ideally I don't take screaming kids outside as I feel it is not fair to the neighbours since I am zoned residential and not commercial meaning my neighbour could be home sleeping if they are on shift work or sick and that is not neighbourly of me to let a screaming kid disturb them.

Once outside what does she do as in does she enjoy playing once there or keep it up. Something as silly as her outside boots have gotten too small to play comfortably could be the problem too.