A classic way of doing so, which is I am always doing, is through Local Users and Groups from Computer Management. You just pick the group to which you want to add user and go from there.
Here is a different way that uses the Net Localgroup command right from the Dos Prompt Window.
1. Open Dos Prompt Window as Administrator. Here is how if you don’t know.
2. To add a user 102nd from domain mhnetwork, type in:
C:\>net localgroup administrators /add mhnetwork\102nd
3. Done, noticed the 102nd user appears in the list?
To see the complete syntax of the command, type in net localgroup /?.
That’s it. Enjoy.