How to add existing users in existing groups in linux and see which user is in which group

Add to group:

[root@288832-web3 ~]# usermod -a -G group1 user1
[root@288832-web3 ~]# usermod -a -G group2 user2

View which group which user belongs:
[root@288832-web3 ~]# groups user1
user1 : user1 apache group1

[root@288832-web3 ~]# groups user2
user2 : user2 apache group2