I'm sure there would be a way to do this using any old encryption library, but in order for Revit to function correctly the family would need to be stored within the project database without encryption. Which means that a user could easily edit and save a copy of the family. Even if you were to monitor the save function and encrypt the family on disk. They can just disable your addin and do whatever they want.
Just how special is this family? If it is sufficiently complex and is driven mostly by the api anyway. You could consider obfuscating the parameter names and formulas or just remove the formulas and use a dynamic updater to calculate and set the parameter values manually, making the family useless without your addin. It all smells like a bad idea though. One of the reasons that collaborative BIM is taking so long to become mainstream is the fear of publicly releasing intelligent product information and design data.
Just how special is this family? If it is sufficiently complex and is driven mostly by the api anyway. You could consider obfuscating the parameter names and formulas or just remove the formulas and use a dynamic updater to calculate and set the parameter values manually, making the family useless without your addin. It all smells like a bad idea though. One of the reasons that collaborative BIM is taking so long to become mainstream is the fear of publicly releasing intelligent product information and design data.