I've been noticing that for newcomers it is fairly difficult to join other active servers, so I tried to join almost all of them and see which were empty and which weren't.
Here's the list:
"Ultra smash bros", password "123", 3 empty
"HellzyBoyII", password "123", 1 empty
"Nem The Nemer's Help Line
" NOTE: The emoji at the end is : D but together. Password "7720", 3 empty
"BuddyCrusade7", no password, 1 or 2 empty
"Guacamostly", "1234", 3 empty
You're welcome