The pi group is the default group created for the pi user. Each user has a matching group name.
- dialout allows access to the serial ports.
- adm is used for system monitoring tasks and gives access to many of the log files in /var/log.
- netdev members can manage network interfaces.
- sudo group members can execute any command (If the goal is to have a lower privileged user, I would not suggest adding your new user to this group. Instead I would add a per command or usename based rule to the sudoers file).
In general you can use google to discover the purpose of a group, by doing something like this: google search, oviously changing the name of the group.
I would suggest that you create you new user without adding them to any additional groups (like the pi group they will be a member of the group with the same name as their username).
And as you find things the new user can not do add the user to specific groups. This is in keeping with the concept of least priviliges.