In Ubuntu, the security model is to invoke sudo command whenever root privilege is required. On the contrary, one needs to login as root using su - command. In fact, only those who are in the wheels group can login as root with su command.
A user not in wheels group cannot login as root:
$ su -
To manually add a user, say unixnme, to wheels group, one needs to edit /etc/group to read something like
Note that root is in the operator group as well. The operator group will have access to devices, so I might as well add to this group as well.
Now, the user can issue
$ su -
commands to login as root.
By the way, if one prefers the Ubuntu way with sudo command, it can be done.
# pkg install -y sudo
Add to sudoer by editing /usr/local/etc/sudoers to read something like
root ALL=(ALL) ALL
unixnme ALL=(ALL) ALL