If you want to add or remove users from the sudo group on Ubuntu, you can follow the below examples,
Add user to Sudo Group

% sudo adduser c2c sudo
Adding user `c2c' to group `sudo' ...
Adding user c2c to group sudo
Done.
If the user already exists in the sudo group make use of sudo adduser command,
% sudo adduser c2c sudo
The user `c2c' is already a member of `sudo'.
Remove user to Sudo Group
In order to remove a user from sudo group you can make use of sudo deluser command,

% sudo deluser c2c sudo
Removing user `c2c' from group `sudo' ...
Done.
Again, if you try to remove a user that does not exist in sudo group you will get below error,
bash|/usr/sbin/deluser: The user `c2c' is not a member of group `sudo'.
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