There are several instances of pike usage during the medieval period, some of them quite 'early.' Part of the trouble comes from separating pike and spear usage, as the spear remained one of the most popular infantry weapons of the medieval period. But if we consider mostly formation-based pike usage then that narrows the field a bit.

There are a few specific areas that pike usage was popular during the medieval period. As is often pointed out, the Scots were adept with the usage of the pike, and there are sources relating to its widespread usage at the battles of Stirling, Falkirk, Bannockburn and so on. Scottish pike usage was particularly dependent on the 'schiltrom' formation, which was essentially a static formation, with the pikes facing outwards and the front rank holding their pikes in the ground to increase their effectiveness. I won't go into the merits and disadvantages of the schiltrom, but suffice it to say that it achieved both stunning successes and catastrophic defeats during the medieval period, particularly against English armies.

Another medieval group that was adept with the mass use of the Pike were the Flemings, or more generally those living in the Low Countries. As early as the reign of Henry II of England (12th Century) the men of Brabant were known so well for their value as infantrymen that Brabançons became a blanket term for such mercenaries. At the battle of the Golden Spurs Fleming pikemen inflicted a heavy defeat on the French knights, widely considered to be the best soldiers at the time.

That gives you some examples of how the pike was used during the medieval period. However it should be noted that there aren't a ton of references to mass pike usage in the early, early middle ages. And throughout the period the mass use of the pike was restricted to more or less the areas I pointed out - pikes were probably common elsewhere but for whatever reason the training and tactics necessary to make them effective were lacking in most other areas. Other weapons that could be equally effective came to be dominant, so its not necessarily indicative of lesser tactics or weapons choice in other regions.

Hope that's helpful! While it's an outdated source, Charles Oman's The Art of War in the Middle Ages does have some good historical detail about the use of the pike in the areas I mentioned. Also perhaps J.F. Verbruggen's The Art of Warfare in Western Europe during the Middle Ages might be helpful.

Hope that answered the question!