Any DAW can import and export plain audio files with individual tracks or mixes, so that's what I've always done. Just make sure they all start at the same time. So, e.g., if you're working together on a song that has no bass on the first verse, then when you send them a bass track, it'll have a bunch of silence at the start--that's fine, and means they can import it and have everything line up exactly right without having to do any guesswork. Usually .wav, but I'm sure mp3 or ogg or whatever would work too.
Maybe if you were trying to do some closer collaboration on the mix, you might need some fancier kind of sharing, but for just contributing a bass track I think that's all you need.
I've only done this a few times, though, I'll defer to anyone with more experience....