When I first started my PhD, I would grab every opportunity to collaborate with others because there weren't many and I wanted to learn as much as possible. However, as I become a senior PhD student, more and more people invited me to collaborate with them which can be quite overwhelming sometimes and I found it hard to turn down these requests. I wonder if there is any way to politely turn down these requests, especially ones from researchers senior to me?
Thank you so much for keeping me in mind for this. Even though the idea sounds exciting, I'm currently swamped with work, and cannot join a new collaboration at this point. I'm very sorry!
Depending on whether you see the chance to collaborate in the future, or you just want to get rid of them: You can also add that they can contact you again at some later point, or even that you will contact them.