Upvote:3
You can find out if you need a visa to visit Canada at the CIC site. Filling in the appropriate answers will tell you that citizens of the Dominican Republic need a visa to visit Canada, unless they have resident visas for the US.
You can apply for such visas. The site above will tell you how. You will also have to ensure that the US visas your guests have are multiple entry. And all of you will need to bring your passports.1
As US passport holders you yourselves can visit Canada without visa of any kind.