You need 3 things to join the crusade: they must be led by a general and not a captain, there must be at least 8 units in the army counting all the general's bodyguard units, and they must be on land - you can't join a crusade while at sea. Put them on land, select the army, and then open the mercenary hiring screen. Down near the bottom left there will be a little button to join the crusade, next to the button that shows the details on the bodyguard unit stats.