There are no real "administrator" and "normal" users (like in Windows). On Linux, you only have one
administrator, "root", who governs all, and a bunch of normal users.
These normal users can be given special access (via a multitude of mechanisms) to certain programs, files, etc if needed. Usually, user groups are created to group users with similar access.
A group of "administrator"-like users is thus possible. If you have such a group, you can add usermod or edit /etc/group and/or /etc/passwd to add users to the group.
If you don't have such a group, you should consider what the user may (not) do, may (not) have access to. Based on that, you can define a proper policy (file permissions, sudo, etc).