BSD Users
A user can refer to either a local user that a real person uses interactively or a system user that is used to set permissions on system processes.
Creating Users
See adduser(8). It can be used interactively.
Privileged Users
Standard practice is that privileged users are in the wheel group. Only these users can su(1) to set their UID to root.
Therefore, to escalate a user's privileges, try
adduser USERNAME wheel
Custom Groups
To create a custom group, see addgroup(8).
addgroup GROUPNAME adduser USERNAME GROUPNAME