How would you take a photo of people who are posing in front of a window? Ideally both the people and the background should be visible.
I usually get either the background being overlighted or the people being near black.
I haven't tried a flash because I want to avoid a big reflection in the window.
You must use a flash to balance the exposure. You can either use your camera's on-board flash, or an external one, it doesn't matter. Depending on your camera (it would help if you offered more information on that part), you should expose (i.e. meter) for the scene outside - as if there were no people, and you simply wanted to take a photo of the outside view - and simply fire the flash (in most cases, TTL flashes are smart enough to figure out what you want to do). Occasionally, you might want to use some flash exposure compensation, for a more natural result (anywhere from -0.7 to -1.7, depending a lot on the given situation)
You have to use a flash, or some other powerful light to light the people in front of the window. If there are many and big windows in the building, you may also use a reflector to guide light towards the people.
But, you have to be careful not to direct the light so you get a reflection in the window. This can be done with just some minor movements form either you, your flash or both. Do you need to stand directly in front of the window? If you can move just some degrees left, right, up or down, reflection will not be a problem.
Chris's suggestion of an off-camera flash is a good one. Another option could be to "cheat" by taking two shots — one with the exposure adjusted for the outside background and one with a longer exposure for the people in the foreground — and combine them using exposure blending.
In fact, you may well want to do both: use a flash (or just static lighting) to brighten up the interior scene a bit, and exposure blending to fix the remaining difference.
Also, if you've been standing directly in front of the window while shooting, try moving a little to the side. Not only will this let you use an on-camera flash if you want (as the other answers note), but it also means you're not shooting directly against the light coming in from the window.
When you shoot directly against the light, all that ends up in the picture is the shadowed side of the subject, so that the entire subject is left dark and flat. Moving slightly off to the side allows the subject to be partially lit by the light coming in from the window, giving it much more contrast. In fact, depending on the effect you're going for, you may find yourself not even needing any fill lights or post-processing tricks at all (although those can still be useful for keeping the contrast from getting too stark).
I'd suggest popping-up your built-in flash (if you don't have an external one) and using a diffuser on it (or making one).
If you have a reflection in the window, you could position yourself so that the model is between your flash and the windows, so the model would prevent the flash from hitting the windows directly. You could also position yourself just far enough so that your flash reflect on the wall and not on the window.
If you don't want to use the flash because of reflections you will need to adjust how your camera is metering the scene. You need to use spot metering on your subject so you camera will expose you subject properly.
If you don't want flash reflections in the window, and you don't plan to use off-camera flash with umbrellas etc, I suggest to turn the strobe's head so that it points at a wall (nearby) or the ceiling. I think the bounce will give you plenty of light, and diffuse its direction at the same time. You'll probably want to set the strobe to maximum power, to get enough light to balance that what comes from outside. If you can still see a reflection of the flashed wall, pick a different wall or turn away from the reflection.
A bit more than probably what you want to bother with, but the motion picture industry does make large neutral density "gels" (flexible as well as rigid) that you can cover windows with. More done with videos as those guys can't use flash, but it is useful to know that they exist.
