I can share with you what I did. Just a note of warning, I only added coffee to half (1/2) the batch. After the first half was bottled I added the coffee to the bottling bucket so only half my bottles would have coffee-porter in them.
I made a porter that was bottled on 07Feb2012, I'll add the ingredients at the bottom to give you a feel of the porter I made. Anywho, I experimented with different cold-brewing techniques (which I found, through reading, is the best way to add coffee at bottling) to get a good coffee flavor, and this is the one I used.
Buy whole roasted coffee beans, I used a Colombian kind with no additional flavors, 12oz bag. The day (or two) before bottling the beer I made the coffee. (you can also use espresso, which I tried but not with beer yet)
1. Grind 3 cups of beans to a coarse consistency, in a coffee grinder
2. Place all the beans is a suitable pitcher/container that can be closed with a lid
3. Add 3 cups of filtered/clean water to beans, close container
4. Place container in fridge or other cold area
5. Let bean mixture infuse for 1-2 days (I did two days when I bottled)
Bottling time, I added corn sugar and bottled half the batch as normal, after the first case was finished
6. Strain and filter (with a coffee filter) the bean mixture, now you have cold brewed coffee
7. Put the coffee in a pot and boil to sterilize, then cool to room temperature
8 Slowly pour some coffee into remaining half of Porter, stir with a sterile spoon
9. Repeat step 8. adding coffee until it tastes right to you. I added all of mine, I love the taste coffee
NOTES: when the final coffee is strained from the beans only half the original amount of liquid was obtained, so I started with 3 cups of whole beans and 3 cups of water, and I obtained 1.5 cups of coffee.
As of 14Feb2012 I've only tried one bottle of porter and one coffee-porter, they are still a little too green/bitter and need more time to mellow out in the bottle but both were good. The coffee-porter tastes like I am drinking black coffee, but not to an overwhelming degree, they both taste very roasted, which is what I wanted, and very rich compared to my first beer. I might cut down on the black patent if I brew this again, and maybe add a hint of vanilla to the coffee-porter bottles, and change the hop type to just one basic English hop.