Shared GND


Sorry for the ignorant question, but how do I connect the grounds of an external power source and the Arduino - do I need aditional components or is the following setup ok?


No extra components and just connect all the grounds together.
If some grounds carry a lot of current then you are better of with a star ground, that is a single point where all the grounds mete and not a chain where one wires goes to each ground in turn.