An umbrella isn't going to be much help if extreme cold, snow, sleet or high winds is your weather woe. But if your main worry is keeping the grill (and the griller) out of the rain, it may do the trick.
There are umbrellas made especially to shelter grills. You can find various designs of grill umbrellas on the market. Typically, they clamp onto the grill's table or fit over it and rest on the ground. Some larger styles have fancier names like "veranda grill canopy."
Grill umbrellas started as a way to provide shade for the fair-weather summer chef. They're also a great way to keep a sudden shower from spoiling your party. They can work well in an extended rain -- as long as there's no wind blowing the water sideways.
If the rain isn't too cold and the guests want to chat with the chef, standard umbrellas can help with the socializing.
No grill umbrella? If rain starts, improvise with the ordinary kind, preferably a large one. Enlisting a friend to hold the umbrella over the grill for the cook can ensure that the inclement weather doesn't end the socializing.
Think an umbrella's not big enough? Keep reading.