You might encounter the need to create a local user, add it to the local admins remotely. Being if the user cannot log in or the local admin is lost.
For the purpose you need to use PSTools which can be downloaded from here.
Once downloaded open a command prompt as Administrator from the server or a machine logged in with a domain administrator and type
PSEXEC \\computername NET USER localadmin <pass> /add
PSEXEC \\computername NET LOCALGROUP Administrators localadmin /ADD
localadmin being the username you wish to use and the <pass> is the user’s password.