First off, you've got the wrong concept for single sign-on (SSO). SSO means that you only have to log on one time and get to all your applications. This is typically done via Windows authentication. You can have reduced sign-on (RSO) if you are authenticating thru another system; but this doesn't, in itself, limit the number of times a person can login - if that's what you're getting at.

Second, you need to look into configuration management tools. I suggest Subversion and TortoiseSVN. This combination has proven to work quite well on small and large, medium-sized teams. There is a built-in merge tool, so team development is quite possible and quite easy. Merging changes won't be your biggest problem, tho. Your biggest problem will be working with different versions of the same module. If you are all working on different modules at once, then you can avoid this problem. If not, there are a few work-arounds that I've found to make life a little easier. Everyone's environment is different, tho. There are too many concepts to discuss here and now, but this should be a start to get you headed down the right path.