Once I accepted a postdoc offer with "A", is it ok to accept an invitation to present with group "B"? 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?
Give the presentation.

There is no problem here (to be honest, I'm struggling to work out why you would think there is). Giving presentations is part of being an academic. Different groups can talk to each other. Different groups with close enough interested should talk to each other.