Join it definitely, the great thing about rugby is that it is a sport for any size
I think that you should definitely join. Nobody is just a rugby player they all need somewhere to start. I was never sporty until I started playing rugby cause I didn't like other sports. I would suggest that you do join because it will help you get better at the sport and being in a team is a good thing. Since you say you aren't in great shape I would tell you that some good workouts are: go running or jogging cause you need the stamina for rugby, do lots of push ups, sit ups cause core strength is important, and pull ups are really great if you have a bar to do them on. The best option is if you have a gym you can go to and use the weights there, if you can, take the weights you can do 8-12 reps with. But not only working out is important. You should watch a lot of videos on YouTube about rugby and watch full games you can find, this way you can learn the rules. Good luck and enjoy playing rugby cause it's an awesome sport!