I recommend using GUI tool called Users Administration. The Users Administration Tool allows you add, delete and modify the existing users and groups in your system. To start

Click on System > Administration > Users and Group

You can also start tool from command line / terminal:
users-admin &
Also you can use usermod command to add user to group. useradd command create a new user.

Add user sweta to group minerals
To add user to the supplemental group user usermod as follows:
usermod -a -G minerals sweta
More info
  1. Howto: Linux add user to group
  2. man useradd
  3. man usermod