Definition #4 in your first link seems to fit this usage of join well.
Today I'm joined by (name)
has the meaning of
Today (name) and I are coming together/meeting
This phrase is often used in radio programs.
It's neither formal nor informal. In an informal conversation with a friend, you can use join. For example:
I'm going to join my friend at the arcade.
Another way of wording that without the use of join could be
I'm going to meet up with my friend at the arcade.
Either sounds fine in an informal conversation.
Your first link has a good example of joined by being used outside the context of radio programs as well:
The police car was soon joined by two ambulances.
That sentence means
The police car and two ambulances came together/met up.