My grandma is from dominican Republic, and I am wondering if she could enter canada with a Dominican Passport, and a U.S. visa.

She needs to apply for visitor's visa at the nearest Canadian Embassy.
Check here Visiting Canada (http://www.cic.gc.ca/english/visit/index.asp)

Having US visa has nothing to do with entering Canada, plus if she enters Canada by crossing US border, she will not be permitted to return to US again even with multiple entry visa.