Just send them a
private message on the forum and ask them if they would RP with you. If they say yes, then you can start talking communicating and come up with a plot together. Yes,
together. An RP takes two people, so, both people should be working on it equally and that includes coming up with a plot. It's not fair to make someone else do all the work.
I suppose you
could ask someone if you can join their RP... but more often then not, they're going to say no. When you create an RP, you tend to decide on how you want it to work (and that includes how many people are playing). So, it would be hard to admit another person into the RP without changing a large majority of the plot that they had already worked out to accommodate them. You see?
If you're still confused about what I mean, try checking out and reading my post:
Creating Your First RPG & Managing It!