I put about 4 oz in the secondary. I took two stalks that I husked with the green bits removed (so really, just the inner "heart") and cut into chunks. I put it in the secondary and racked on top of it for 5 days.
Basically, I used my dry-hopping technique on it.
The problem I had with it was like a dry-hop, the aroma was pronounced - too much so. The beer didn't have any aroma characteristic except the sharp lemon smell. Also, unlike other fruit additions I've tried I didn't think the lemongrass oils dissolved very well in the beer. It was like a black and tan with beer and lemon juice.
I'm sure I could have engineered a way around it, but I found a recipe here (At the bottom of the first page: http://www.homebrewtalk.com/f76/lemongrass-rye-133081/
) that called for the lemongrass in the mash and boil, so I tried that. It came out so well I abandoned the secondary idea.