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.
I'm not good at acting in a group.
If you go to that country, you have to be careful about terrorist groups.
Koichi broke out ahead of the crowd at the end of the race and won the Honolulu Marathon.