This is a free to use Drawpile server.
To host a session here, launch DrawPile and select the Remote option
in the Host dialog and enter drawpile.invertex.xyz in the text box.
Sessions here can be set to be Persistent in their Session Settings, which means even after everyone leaves, the drawings will remain, and people can hop back into the sessions at any time and keep adding to it! Making for an ever-changing canvas!
Be kind and have fun!