Details are very important here: 

I interviewed for postdoc positions at 2 labs (same university), and received offers from both. 

I accepted the offer from lab "A", and when I declined the offer from "B", the PI told me that they were sorry about my decision, and that they would still like to invite me make a presentation for their lab. I thanked him for the invitation and agreed to make the presentation. 

The two groups ("A" and "B") do not interact (they work on different fields), so this is essentially a matter of etiquette. 

Is it ok if I make the aforementioned presentation now that I started my appointment with "A"? Should I inform the PI of "A" that I will go present with group "B"? Or should I just decline the invitation to present?