----------------YOU HAVE TO BE A SCRATCHER------------- If you are not a scratcher you can't access CLOUD VARIABLES so you can't connect with other scratchers. To become a scratcher you have to be pretty active for 2 weeks. :)
Up to 300 players can play at once but that's just because this engine uses CLONES and there can only be 300 of them at a time. There is a stamped version which can have way more than 300 players at once but it is more laggy. Stamped Version: https://scratch.mit.edu/projects/362631829