The GUI way
Right click on My Computer
or type compmgmt.msc in start menu.
from computer management select local users and groups
find the user you want to add to a group and select properties
Select the Member Of tab click Add and select the group you want to add them to.
A quicker and batch scriptable way of doing this is
net localgroup GROUPNAME /add paul
The batch script adds a user called itadmin adds it to the Administrators group and removes it from the users local group all in a fraction of the time it would take using the GUI.
net user itadmin P@ssw0rd /add /fullname:itadmin /expires:NEVER /passwordchg:no /usercomment:”Administrator account”
net localgroup administrators itadmin /add
net localgroup users itadmin /delete