Generally, anyone can come visit for up to 6 months in a row. To legally be employed, you need a work visa of some sort. For a short term gig, in the old days, a letter from the employer was sufficient. I was on flying back from Canada with a man doing just as recently as ten years ago. I would search for the different types of work visas. On the other side of the cost spectrum, there are entire businesses based around sorting these sorts of things out. You could search for an immigration consultant.