This can be opened with an app called Terraria Save Manager, A windows app that can open terraria saves. The steam version automatically saves your world data in a. Terraria isnt a heavy file and it wont slow down your device or take up much space. This is a great option for the family if youre interested in playing on devices you already own instead of purchasing an expensive console. Both parties will have to purchase the game to accomplish this so make sure theyre installed and it should be a breeze from there.
You should see an open chat window with the invite. If the notification has expired, open the Steam overlay manually using the key combination 'Shift + Tab' (Or whatever you have set the keybind to).
Enter a password or leave blank if you don't want the server to be password protected.