Email etiquette regarding research collaboration?

I am a graduate student who is seeking to collaborate with a researcher outside of my university in addition to my senior supervisor at my university for my dissertation. I have worked with this other researcher outside of my university in the capacity as a research assistant for a couple of years. I recently sent him, along with his colleague with whom I am also familiar, an email regarding the possibility of collaborating for my dissertation. This email was sent 10 days ago and I still have not received a response regarding whether or not either of them would be interested in being on my dissertation committee. What is the etiquette here regarding how long I should rate for a response? Is it okay to send a follow-up email at some point if I do not receive a response, or should I consider the no response to indicate that they are not interested in being involved? If I should send a follow-up email what should it say? Any advice is appreciated.