To Come Together
To Gather, To Be Gathered
intransitive verb, godan verb
I bet you remember 集める which means "to collect." This is similar to that, except you're not doing the action of collecting, it's just happening. Instead of "to collect" it's to come together. You can remember this because a herd of Marus (the cat) will come together into one place if they sense a box there.
あつまる
The reading is the same as 集める so if you know that word you'll know how to read this word as well.
クラウドファンディングで、こんなにたくさんのお金が集まりました。
This much money was gathered by crowdfunding.
社員旅行は、全社員が集まるいいき会になります。
The company retreat gives all the workers a chance to come together.
見て！あそこにヤンキーたちがたくさん集まってるよ。
Look! A group of miscreants are gathering over there!