A walk-on can play on the team and go to matches. However this isn't always the case because usually the scholarship players start in the line-up and walk ons usually get to just practice with the team. A friend of mine joined a D1 tennis team as a walk-on and actually ended up trying out and was placed low in the starting line-up. She didn't get a scholarship though...which she should have, but coaches won't guarantee you a scholarship as a walk-on...it's not fair if you play in the starting line-up, but some coaches are stingy.
Your best bet would be getting into a school where a scholarship or spot on the team is offered and basically guaranteed. Trying out for a D1 team is very difficult at a number of schools and many have failed at doing so. However, it's possible, and if you have your heart set on one school and you haven't been offered a scholarship, walking onto the team is your only option if you want to play tennis there.
Walk-on athletes have to get in the school like anyone else. The coach could potentially give a good word for you but that's not likely especially if he hasn't offered you a scholarship. It's possible though, especially since coaches are limited to a certain number of scholarship players. Again, contact the coaches at the school you are interested in. They will be able to help you a lot with these issues.