Trying not to spoil anything from the comics... but I don't think this spoils anything...
The prison was a secure place, except they ran into the Governor nearby. Without the governor, they could have stayed at the prison indefinitely because they had lots of room to garden and it was pretty easy to secure from the walkers.
Alexandria looks to be a gated community. That's why it would be better than your average neighborhood. IF you sweep the gated community, you have a secure neighborhood AND until the population of your camp gets too big there will be lots of houses to spread out for everyone. The gated communities usually have large houses too, so even if you shared a house with someone you'd have lots of room especially since nobody really has a lot of personal possessions anymore.
Alexandria would have the same problem as the prison, though... in that once another group finds out about it there really is only two choices... that other group wants to join and is one you want to invite in... OR they want in, and you don't want them and you have to fight it out. So you'd be really careful and selective about who you invite back to your secured camp. Rick and company weren't at the prison long enough to be going around inviting others in, though they did take in a lot of renegades from Woodbury for a while. Presumably the Alexandria camp has been around a while from the way Aaron was talking about having "done this before" meaning, Rick's group wasn't the first group they found and scouted and asked to come see the neighborhood.