when sharing source code to more than one party, should I notify them that others also were interested
One possible course of action could be to make the code publicly available on your website, or some public repository system, e.g. github. That gives you no control over the distribution, but provides other important things like public appreciation and (in case of github) widely recognised timestamp.

Since this is your code, you have the right to promote yourself by mentioning that it is widely used. However, it may be not a good idea to share the information about someone else's current research interests, without asking them first. Some areas can be very competitive, and it is better to be safe than sorry. You could ask your "users" if they agree to be mentioned as such.