C'mon in - the water's fine!
The internet has been a paradigm shift for many things, the realm of making music not the least! Just think - coupled with the capabilities of home recording, it's now entirely possible to create a "virtual band" made up of members from around the world. And you don't even have to leave the comfort of your own living room to do it!
There are myriads of on-line collaboration sites where musicians gather for exactly that purpose. Someone tracks an idea for a song, and asks for help on other parts. It doesn't even necessarily have to be a song - many writers post potential lyrics, for example.
I mentioned on the home page that most of the songs here were created at Echo Project Studio, by the virtual band we call LimberTwig. EPS is one such site - we do it "behind the scenes" as it were, taking our time, working with what we each have to offer, and getting it as right as we can. Even when we're not actively working on a song - and we always have several being worked on - we take the time to trade banter and just enjoy each others' company. It's kind of like Facebook - for musicians.
An excellent open-membership site is Nick Tesla's Musician's Collaboration Studio. There are many more...
Most, if not all, of these types of sites require "registration" - no worries, it's usually free. The key is finding one where you're comfy with the existing membership, and then joining in the fun!
In my experience, the skill levels of those who actively participate on these sites is amazing. Most are reasonably experienced musicians - many who've grown tired of the rigors of live gigging but still want to play, others who may be "between bands", and still others who do both live and on-line work.
The sites I'm familiar with work on the "forum" model, with different threads going simultaneously for different topics and songs. If you're not familiar with the forum concept, you'll find it easy. It's a lot like a blog, only more organized.
It usually goes something like this: let's say I've got a song that's well under way, but needs a lead guitar track to bring it home. I could "advertise" that need on the forum thread, and provide the latest mix for prospective lead players to track to. Anyone who's interested could then track an "audition" take, and post that. Having listened to the (hopefully!) various auditions, I can select the part I like best...
There are a few "requirements" that are peculiar to the world of on-line collab'ing - most notably, you'll need a place to store your files where others can get them. Many will allow you to "attach" your tracks to a posting, obviating the need for independant storage. Even if that's not an option at your site-of-choice, there are tons of storage sites available on the 'net, including many free ones that are perfectly adequate for this kind of use.
You'll need some kind of "file-conversion" utility for trading tracks - .wav files are just too big for practical use. For those of us who work on Windows-based platforms, .wma is a good choice for "working" tracks. Mp3 is not a good option here - for whatever reason, the conversion to .mp3 adds an unpredictable number of milliseconds worth of "dead air" to the beginning of the file, which makes it difficult to sync up with the rest of the tracks. Once the "final mix" is completed that's no longer a concern, and .mp3 is a perfectly acceptable medium. You'll notice that I've used it for the songs on this site. You'll need a "lossless" compression codec for transferring your final take. .Ape (Monkey's Audio) is the method of choice in the circles I inhabit. And finally, do a Google search for "dbPowerAmp" - it's a freeware file conversion utility that integrates seamlessly with Windows. I'm sure there are similar apps for Mac users - but I work in the Windows world, so don't know what those are.
If you're not familiar with (or not accustomed to) playing with a click-track - learn it, and learn to love it! Since you'll be playing your part alone in your home studio, there's no opportunity for the kind of interaction you get between band members in a live setting, and no visual cues. So strict adherence to "the clock" is a must, for parts to sync correctly. This is doubly important if you plan to use any midi or synth tracks - those things are by their very nature dependant on the clock (and yes, that can be adapted, too, but it's a laborious process in most cases).
And finally, until you get pretty accustomed to the interaction at your site of choice, take a bit of care in "how" you say things. Remember that you're not really "talking", you're typing. It's all too easy to post something with perfectly good intent, only to have it taken completely otherwise by the reader. You'll be making new friends - no sense in losing one over what's most likely a misunderstanding caused by a poor choice of words...