Using Command Line how to make the user an Administrator
sudo dscl . -merge /Groups/admin GroupMembership username


where ‘username’ is the username of the user you would like to make an admin.

sudo since making changes like this requires elevated privileges.
dscl is the Directory Service command line utility.
. is the local machine.
-merge allows you to add a new key to a record path.
/Groups/admin is the record path for the key.
GroupMembership is the key you would like to assign a value.
username is the value for the key.