I can't speak for the entire medieval era in general, but during the first few Crusades women definitely accompanied the armies. There was the usual assortment of prostitutes, cooks, cleaners, etc. that were all grouped into the broad term of "camp followers", but many lords and knights also brought along their wives, most notably when King Louis VII of France was accompanied by his wife Eleanor of Aquitaine during the Second Crusade, who reportedly showed great personal bravery in inspiring the troops in moments of hardship, and also keeping the other women calm when they were ambushed repeatedly in the mountains of Anatolia. During the First Crusade the soldiers formed a large defensive perimeter around the women accompanying their army when they were ambushed by a Seljuk army at Dorylaeum, and a century later during the Third Crusade many women aided the soldiers during the Siege of Acre by digging trenches and hurling missiles at the city walls (a sight which shocked and disgusted many Muslim observers in the city).

There are a few occurrences during the Crusades of the more religiously 'devout' leaders of the armies expelling women from the army while it was on the march, as a sign of pure-hearted devotion to God and the cause of the crusade. The accusation that many of the women were merely prostitutes were thrown around, but in most cases that wasn't true.

Source: God's War: A New History of the Crusades by Christopher Tyreman