If you do not want the flash to show in your photos, you either get it off camera or bounce it off a wall. Check the room and see how tall is the ceiling then direct your flash upwards (A-mode – TTL +0.7). I would suggest a tripod or at least a speed of 1/125. The minimum f stop should be 5.6 for group shots. Apertures of f8 or f11 will need much more flash power and will completely darken backgrounds at indoor shots.
Sometimes I shoot indoor groups at f4, 1/125, ISO 800+ and I get everyone in focus due to my distance from them. Also the whole photo looks well exposed. The first thing you need to do is understand how camera and flash works. P for “Professional” is not the way to go on most situations especially for indoor portraiture.