Adding handholds is smart insurance against falls, and gives your passengers peace of mind.
There are all kinds of reasons to add a handhold somewhere on your boat. In my case, I wanted to have a place for people in the rear-facing seat of my runabout to be able to hold on while I execute my standard daredevil maneuvers. Mounting a handle is the same as mounting any piece of deck hardware. Keep in mind that you're basically asking people to fall on this thing with their full weight in an emergency. Having a loose or untrustworthy handle to grab is worse than having none at all. The last thing you want is someone to fall because your poorly installed handle gave them a false sense of security.
The handle I chose had threaded studs welded to it. If you have options, make sure to use thru-bolts rather than screws, which are more likely to pull out, especially in fiberglass.
Once you know where the handle will go, make sure the area behind the fiberglass is accessible, and there's nothing in the way. You'll want to mount a backing plate or block there — made of hardwood or, more commonly, marine-grade plywood, painted or (better) clear-coated with a two-part epoxy like West System. The idea is to prevent the fasteners from pulling out when someone yanks on the handle, and to do that, we want to spread the load as much as possible. You'll also want to figure out whether you're drilling through solid fiberglass or into an area with core material. If the area is cored, you'll need to take a few extra steps — see "Install Boat Deck Fittings". If in doubt, ask the manufacturer about the location, or post a question on an owner's forum.
Once you've got your location marked out, carefully drill holes in the fiberglass and backing plate. Make sure to seal the inside of the new holes with epoxy once you've drilled them. The fasteners should go through the fiberglass and backing plate, and be finished off with large stainless flat washers and Nylon locking nuts. After you're sure everything will fit, take it all apart again, drill a slightly countersunk hole in the glass, and run a bead of bedding compound around the holes before putting it back together. Don't worry about bedding any of the other surfaces — the object is to keep water from getting in through the holes. If it gets in anyway, the epoxy sealing the inside of the holes will keep it out of the fiberglass, but it needs to have an exit. After the bedding compound dries, you should be good to go!