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).