when sharing source code to more than one party, should I notify them that others also were interested A few weeks after my work was published in a conference, I received an email from a research group from another university asking me to provide my source code. The email was very polite and explained the reasons for the request, i.e. the direction of their research. There was really no reason to deny them. However, yesterday (ca. 6 weeks after the first request), I received a similar request from another group asking for the same code to conduct research in the same field as the first group. (To clarify, I'm in CS, but both groups work in statistics) I see no reason to deny them either. 

But should I explicitly state that the code was already asked for and provided to another research group? If so, should I also notify the first group?

I found myself in uncharted territory since I haven't expected anyone to be interested in that code, let alone to receive two requests. Also, this situation has potential to escalate into misunderstanding(s), if information is withheld or provided, but misinterpreted.

PS: I haven't talked yet to my adviser, as he is on vacation and I really don't intend to bother him with this matter.
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.