Pump & Dump communities. Are they legal?
Legality depends largely on jurisdiction, and you didn't specify one.  In addition, the regulation of cryptocurrencies is constantly in flux, and even in a given jurisdiction it's difficult to say which laws apply to them and which don't.  It also may be in question whether fiat money is involved, or are these scammers selling their altcoins for bitcoin.

As far as I'm concerned, this looks like it is illegal, or at the very least should be.  Whether it actually is, is impossible for me to say.  If you want a more definitive answer, I recommend consulting a lawyer in the jurisdiction you are concerned with.