For this launch, the two guys will remain at the Station for a while (they have not yet figured out how long) and then they will come back down in it, to a splashdown in the ocean. Assuming this all goes well, the capsule and system will be declared operational and as many as seven astronauts can ride it at a time. If launches are frequent enough, that could mean that some go up and others use that capsule to come back down - as they do now with the Russian capsules.
The routine now is that there are enough Soyuz capsules docked at the Station to serve as "lifeboats" in case the crew has to abandon ship due to emergency. Soyuz can carry three, so there are always a multiple of three astro/cosmo-nauts there (always at least one Russian and one American), and they obviously then have one or two Soyuz craft. When it's time for a crew change, three come up and three go down, using the oldest Soyuz docked there for the re-entry.
That pattern will likely remain with Dragon and also the Boeing craft when it gets operational. There could conceivably be three types of capsule docked there at once - and larger Station crews. Even tourists!
