How safe is it with Grizzly bears in that area?
As safe as you make it.
Don't fool yourself, bears are there, and they will attack you if they feel they are being threatened. The Canadian Rockies are wild and full of large predators, it's not not a zoo or game preserve, the danger is real. You must understand that you are in their territory, the most important thing to do is to properly educate yourself for travel in bear country. Treat the bears and wildlife with respect, be wise, and you will be fine.
Thousands of people hike and camp in those mountains everyday from all around the world, notices are posted at trail heads and advisories of bear activity or aggressive animals are always available at the visitor centre and often online on the park websites. The Parks Canada website also provides a page for educating yourself on Bears in the Mountain National Parks, I suggest you read it.
In addition to that link there are many other questions that address bears on this site, so I suggest you browse those as well.
As long as you go into the backcountry with at least one can of bear spray, something to hang a food cache with (if you're staying at any campgrounds that don't have caches or bear boxes, which most do), and a good amount of bear wisdom, then you'll be plenty prepared to enjoy your trip and be safe.