There were many suggestions, like gathering, meeting, collctables, assembly...but it seemed there was no specific name people call it.
Someone looked up it on the web (thanks!) and he found Wiki calls it a "Cat Colony" .
There was another one who answered me, she explained "a group of cats is called a "cllowder", but nobody really uses that word". Seems it's not common subject in West. Why? Cats in west don't have Syukai ??
I got other answer from a Spanish, and she said it was not rare to see cats doing that in Spain. I didn't ask her if there were any name in Spanish. Hmm. :/
It's interesting, some countries (culture) have the name and some don't. Especially it's relevant to cats.
What about モミモミ攻撃？ I believe I saw the English phrase somewhere but don't remember the word... the one pushing (kneading?) something with it's front paw.
It has been a while since the last 4koma. I drew about Kindle 4koma book, and thought it should have been first. But I changed my mind because I talked some about Cat's gathering on the web today. :) (Yesterday, to be precise)