Adding a user to an existing group may give an error:
useradd: user 'name' already exists
where name is the username. An example of this command should look similar to
sudo useradd -G sambashare danny where sambashare is the group and danny is the username.
In the above case usermod should be used instead of useradd. This is because useradd creates and adds the user but if the user already exists then it needs to be modified. A modified example of above should look like this:
sudo usermod -a -G sambashare danny