My coffee addition was a little different. I actually cold brewed the espresso overnight and added it at bottling/kegging time. I took my batch volume and determined what 3% of that was, which determined the amount of cold brewed coffee that I was adding. It achieved great aroma and a prominent coffee taste, which was what I was looking for.
I have heard of people steeping their beans in the beer. A local brewery suggested that I steep the beans in the batch at lager temperatures for a week, which is what they do. But I've seen others just plain steep at fermentation temps. Before I decided on what I wanted to do, I did a test in water with the beans. I took a mason jar and cold steeped some beans in water for a week to see what happens (scaled down for the amount of water in the jar). The aroma was good but the taste was a bit subdued for what I was looking for. I guess it depends on what you want to achieve.
I think the most important thing with coffee additions in a RIS, is that the beer is finished. I don't just mean fermentation, but rather it's aged enough before adding the coffee. I went two and a half months before adding the espresso.