I'm pretty sure he explained it in the latest episode. the short answer is that they were getting spammed with join requests. 

Jack also mentioned how to get an invite. you should check out the episode.  

 Thanks. Need to just get off my ass and set up a hot wallet. Been meaning to for at least a year or two but it’s been a pretty low priority. 

 Muun was wildly easy to set up. No invite needed. Took me less than a minute. Didn’t expect it to be that easy!