Selena, is it a fast song? If so, you can be moving around while you play it. If you're moving, nobody will notice if you're shaking a bit. And once you get into it for a few bars, you'll feel better and more at ease. The main thing is that in order to feel comfortable, you must know your material. Since you already know how to play the song, you're more than half-way there already! Just keep practicing it until you feel totally at ease with that song. Then when you agree to sit in, you'll at least feel comfortable with the material.
Then just move to the groove. After you get that first time under your belt, it will get much easier. And don't worry about messing up. Everybody messes up occasionally. If you DO hit a bad note, don't freak out. Just keep playing like nothing happened. Most people in the audience won't even realize there was a mistake. But you'll be surprised to find that if you know the song well, you probably won't even MAKE a mistake. Instead of worrying about messing up or making a mistake, just try to get yourself into the proper mind-set. Tell yourself, "This is going to be fun!" Because really, if you stop and think about it, it IS fun. You like playing the bass, and you know your material, so keep telling yourself "This is going to be a blast".
And don't worry about playing in front of the other bassist. Experienced bassists are usually very supportive of people just starting to play out. For the most part, bassists are like a big family - supportive of each other. He obviously thinks you can handle it, or he wouldn't ask you to sit in. Just do it! It'll be fun, and you'll have gotten over that "first-time" hurdle.
