Pump & Dump communities. Are they legal? There are several communities that allow anybody take part in their "movements".
They say, for example, that at 7pm GMT they will announce an altcoin name and people would rapidly buy this coin on a specified early exchange (say, Bittrex).

When it's time to go, they disclose the coin name and people participating start to buy it. In consequence, the price of this coin rapidly rise. And after some short period of time it does down. Therefore, those who managed to buy it early and sell it in time, get profits.

These communities often call it as "Pump". 

I read on Wikipedia about Pump &amp; Dump scheme. It's illegal on security market (as I understand). Although, in my opinion these communities do not really do Pump &amp; Dump as described on Wikipedia (from my point of view) as they do not make "false and misleading positive statements" about pumped coins.

The only thing that looks like described on Wikipedia scheme (again in my opinion) is that admins of some of such communities ask other participants to "promote" pumping coins on different chats/forums.

My questions are:
1) Do organizers (admins) of such communities do illegal things?
2) Do participants do illegal things when they:
  a) "promote" coin
  b) do not promote coin and just participate
3) What are consequences for those who could be determined as the one who do illegal things?
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.