Are random people allowed to enter your backyard without asking?

I caught a glimpse someone wearing those orange traffic safety clothing running/fast walking out of my backyard when I was in the kitchen, I ran to my room which overlooks the front of the house and saw her walk straight across the street and speaking with people across the street in a plaza where it seems to be undergoing construction... I noticed its a plumbing company since there are 2 vans blocking traffic in the parking lot where they are talking. Are these companies allowed to send people into other people's backyards without permission?