My 2003 VTX1800 with Dawg and trailer at Clackamas River Oregon. @25000miles

Yes the family and friends are there to relax and have fun. A photographer at any event should strive to be unobtrusive... and a flash is very obtrusive. But..... as a photographer you have to get the shots. I make it a habit to never publish or show a bad/embarrassing photo of a subject. People see a bad expression once and are uneasy and afraid of you instantly.

I shoot a lot of Boudoir photos of brides before their weddings. not all of them are svelte model types. Actually none of them are. But the job is to make them look as attractive as possible. So you have to think while shooting. Use the angles and cropping that accent the things that you want to show. Hide the things you don't with props, scarves, angles. I have failed in some cases. But limiting to head and shoulders shots with good fill flash will rarely let you down. That's where the person's personality is. That's why we have zooms! Amazon has books galore at great prices. Jeff Smith is a great portraiture author. Good Shooting!

I find that using a little fill flash is very helpful. Auto mode on the camera is your enemy here. Shooting totally natural light is highly over-rated for casual setting where you can't pose the subjects. shooting wide open at f2.8 or f1.4 requires precise focusing and gives a very shallow depth of field. If you want to flatter your subjects instead of creating pictures with raccoon eyes and dark shadows on the faces you should use fill-flash. I usually use my hot shoe flash pointed up to the ceiling at a 30-45deg angle. I also use a Stofen type diffuser on the flash to get the softened light with faint shadows. They are on Ebay very cheap(as low as $1.50 with free shipping).
With the flash adjusted down to -1 f-stop you can still get natural looking photos but with shining eyes. This is done in the flash menu and just takes a second. Experiment with the camera in Shutter priority Mode(1/60th sec) and aperture priority mode. Set your ISO turned up to just below where the noise starts (ISO 800 is good for your Nikon) you can get some bright natural looking photos that flatter the guests.
If you only have an built in flash you can still set it to do fill-in (-.5 to -1 f-stop) and at least brighten the shadows and put some catch-lights in the eyes.
Hold steady! Use a tripod or brace yourself against something. Use shake-reduction on the camera.
Good shooting.
Remember if you are serious take control of the shoot, even at home.

@julian

Sounds good to me. You are having fun. Getting intimate with the things of interest and beauty around you. Whats wrong with that? I know guys that bought $500,000 motor homes and go on one trip per year to sit in a KOA campground just for the socializing.

Don't do it. I shoot weddings. about 10 per year. It takes a lot of work and experience. I am one of the "budget" shooters that the Pro's hate. But my clients can't afford $2000 for a wedding shoot.
I typically get 900 photos of which I whittle down to about 150 for final burning to CD for the couple. The 6 hour wedding takes another 30 hours editing. I have a backup camera and lenses(accidents do happen), insurance, portable studio gear, computers and software to do a professional job on a tight budget. My shooting partner is an experienced lady photographer who can get all the intimate moments of the bride and her entourage getting ready.
There are mandatory posed shots and low-light semi-candids required even if you are doing a freeby job. Without 2 shooters you will miss the little things that are special to brides. The dressing, putting on the gown, hair, makeup..oh and while that's going on the groom is getting ready with his man-folk. It's mostly about the bride by the way but... the old folk that flew in from out of town, The little kids all dressed up, moms crying, the brides maids, the darling flower girls and ring-bearer.
Do your grandson a huge favor and chip in on a photographer. There are tons of semi-pro's out there that will shoot for $100-$150/hr and cover the wedding adequately for a nice scrap book of the wedding that covers all the important events. I didn't mention how tired I am after a wedding. it's very hard work for an old guy. You have noble intentions but be truly noble and help get a photographer so that event will be covered adequately.