No, they are not invisible; you can see them, but you would probably not recognize them as time travelers since they look just like any normal person. Even if one of them told you they were a time traveler, you would not believe them anyway, even if you observed their equipment.
Earlier this year, I was asked a similar question from a colleague, a new member of our team. He asked, “is it okay to talk to people? Will I be arrested, or will our project be compromised if they find out who we are?” The answer is yes, it is okay to talk to whomever you want, and you can tell them what we are doing… it really doesn’t matter, because no one will believe you; they will discount you and then forget about it. And no, the project will not be compromised simply by talking to people during the project. However, interacting with people will most certainly create a future that is different from the one you left. The changes can be so slight that it’s never noticed, or, the changes could be disastrous. The mathematical model we use predicts an acceptable risk as long as the project is properly planned, and we follow predetermined guidelines to help reduce the risk as much as possible. But simply talking to people and interacting with them poses only a slight risk, which we accept as part of our work.