You can't get the nearest player(@p) in a command block with essentials alone. However, you can use minecraft methods to make the player execute the command themselves.
For example: /give <your name> oak_sign{BlockEntityTag:{Text1:'{"text":"","clickEvent":{"action":"run_command","value":"warp JoelArena"}}',Text2:'{"text":"Warp to Arena"}',id:"minecraft:sign"},display:{Name:'{"italic":false,"text":"Place Me"}'}}
When a player right clicks the sign and the player has the permissions to run the command /warp JoelArena, it will execute the command. This is a bit of a cluncky method, however, it would work with no extra plugins.