There are a few ways to travel with your active Sim to different worlds.
You can Press the M key, use the phone, or zoom out until you see the "Travel Sign". A little walk-through is below:
With your active Sim for example, the young man, you can use the phone and select Travel.
Or - zoom out all the way with the mouse scroll wheel, until the "Travel Sign" appears
- Using the scroll wheel - Scroll away from you is zooming in. Scrolling towards yourself, zooms out.
- Using the keyboard - Press "x" to zoom out, "z" to zoom in.
This will take you to the Map screen. Click the World Select button on the upper left corner to choose Willow Creek.
Click on the Family's house. (In my example, I click the Goth family's house) A Plane button should appear in the lower left of the map screen. Click it to confirm the travel plans.
After that your active Sim should be on the Family's lot to visit them.
To go home - Click the tiny house icon on your Active Sim's photo.