[@Pie Flavor]
its not really a binding as much as a summoning type thing
once they appear they would be that witch's familiar since that creature would be linked
A familiar is not capable of their own magic, they are more of a magnifying agent. Though some are capable of witch detection (like Malcolm's) and all can speak. 

basically, you perform a ritual, at the end of it you get your creature. It will be slightly painful as the link happens that bonds witch and creature, but yea.