Rendezvous is an appointment, i.e. the arrangement to meet rather than the act of meeting itself. So you wouldn't usually say something like “I'm rendezvousing with a friend right now”, because the immediacy of “right now” (your friend is supposed to be here) contradicts the potentiality of “rendezvous” (you don't know whether your friend has arrived). If you want to say that you have an appointment with a friend, it would be more idiomatic to say “I have a rendezvous with a friend right now” (you have an appointment, but you don't know whether your friend has arrived yet).
Rendezvous is not a very common word: “I'm meeting with a friend right now” would be the most common way to phrase this. The word rendezvous is a little highbrow and old-fashioned, and it's far more often a noun than a verb.
“I had a rendezvous with a friend” (you two had agreed to meet in a particular place at a particular time, or at least in a particular place during a particular time interval) is fine. So is “I rendezvoused with my friend” (we met at the time and place that we'd agreed). Rendezvous tends to focus on the meeting place; it can also be used to refer to a meeting spot even if there is no particular agreement to meet, e.g. “this café is a popular rendezvous for artists”.
Rendezvous is also used in the sense of a meeting actually taking place in other contexts that are about objects or abstract groups of people meeting. In this sense, it isn't particularly old-fashioned, and the verb doesn't sound more awkward than the noun. For example “the two armies rendezvoused on the plain” (they came to the same place and joined forces as arranged) or “the submarine will rendezvous with the battleship tomorrow” (they'll navigate to the same place and exchange personnel or supplies as arranged).
Rendezvous comes from the French word rendez-vous, and has kept almost the same spelling (only losing its hyphen) and a somewhat similar pronunciation (I think ran-dey-voo is more common than ran-duh-voo). The usual plural is identical to the singular, like in French. Rendezvouses also exists, but it's uncommon.
