Group
Crowd
noun
A group that has collected together is a bunch of people. It is a group or crowd.
しゅうだん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
集団で行動するのが苦手なんです。
I'm not good at acting in a group.
その国に行くなら、テロリスト集団に気をつけなくちゃいけませんよ。
If you go to that country, you have to be careful about terrorist groups.
コウイチは終盤で集団を抜け出し、ホノルルマラソンで優勝した。
Koichi broke out ahead of the crowd at the end of the race and won the Honolulu Marathon.