Group
Crowd
noun
A group that has collected together is a bunch of people. It is a group or crowd.
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.
集団でイジメをするなんて、最低ですよ！
Bullying someone as a group is disgusting!