I'm a Canadian citizen tour guide that will be picking up a group in the US and touring them for a week in the US before we enter Canada to finish the trip for another week in Canada. Do I require any Visa for such cross-border work? My employer is a US travel company.
As a Canadian, entering as a business visitor you do not need a visa. While I can't find official documentation for this particular case, you can find the following on the How Do I Enter the United States as a Commercial Truck Driver page:
Canadian citizens entering the United States as visitors for business do not require either a passport or a visa. However, each applicant for admission is required to satisfy the inspecting officer of his or her citizenship. An oral declaration may be accepted or the inspecting officer may require supporting documentation, for example, a birth certificate, certificate of citizenship or a passport (valid or expired). In addition, all travelers should carry some form of photo-identification.
That's crystal clear. Now the only question is, are you a business visitor? Well, let's read the law § Sec. 214.2(b) Visitors has this under NAFTA:
tourism personnel (tour and travel agents, tour guides or tour operators) attending or participating in conventions or conducting a tour that has begun in the territory of another Party. (The tour may begin in the United States; but must terminate in foreign territory, and a significant portion of the tour must be conducted in foreign territory. In such a case, an operator may enter the United States with an empty conveyance and a tour guide may enter on his or her own and join the conveyance.)
Looks like your case is explicitly covered and yes, you are a business visitor. Go ahead, good luck! (The same page also tells you don't need a visa but it's written in unreadable byzantine legalese so I found the CBP truck driver page better to explain that.)
Note that the relevant Canadian guide about Cross Border Movement of Business Persons misses the section in () above and as such is completely misguiding. However, the law certainly trumps any informational pages.