In the past few months my scrum team has doubled in developer size. We started out with four developers, a Scrum Master and a Product Owner and have since added four more developers (two full-timers and two co-ops, who are full time until the end of December).
Recently we discussed how we no longer knew exactly what each other was doing. We might know generally what part of the product they were working on (through standups), but now had very little idea of how they implemented technical details. With standup time being the same (15 minutes) and the number of people who had to speak doubling, there was no way we could fit in extra discussion. We threw around having occasional code reviews, but anything that isn't regularly scheduled tends to fall by the wayside pretty quickly.
Our final solution was for the whole team to show up to our biweekly demo an hour early and go over what we changed on the projector. It's a perfect time to show completed code, do knowledge transfer AND prepare for the demo.