#1. put the username on the white-list.txt file
#2. you might want to add this username to your permissions.yml and give them a rank (i.e. default, Guest, Member, VIP, Moderator, Admin, etc).
#3. optionally, if this is a trusted user, you could also add their username to the op.txt. If they are in this file, then they will have ALL permissions (except for negated nodes).... Then you wouldn't have to go thru the hassle of creating a permissions.yml.
#4. if the Mojang servers go down, you could also edit the server.properties file and change online-mode=true to online-mode=false. This will allow your players to connect when the Mojang authentication servers go down... Just remember to change it back to online-mode=true... once the authentication servers come back online.
Hopefully this helps... It's kinda an odd question... So i might be interpreting it incorrectly. Good luck!