You cannot use "reference" to fill in the blank here.
"When you want to know about IT, ask John. He is the ___ in this topic within our group".
But you can "refer" to this person, who is an ___ in this topic.
A formal and most commonly used word would be:
Expert — ODO
A person who is very knowledgeable about or skilful in a particular area
"an expert in health care"
synonyms: specialist, authority, pundit, guru, oracle; adept, maestro, virtuoso, master, past master, professional, genius, wizard; connoisseur, aficionado, one of the cognoscenti, cognoscente, doyen, savant
If you want something less formal:
Go-to — M-W
always helpful : producing desired results or information when needed
relied on for expert knowledge or skill
"the company's go-to guy"
"He's the go-to guy in the office for tax information"
Even more informal, especially if it's the IT field:
Ninja — ODO
1.1 [OFTEN WITH MODIFIER] informal A person who excels in a particular skill or activity:
"My goal as marketing ninja is to get Thomas to spend less time in London, and more time on a plane, visiting his other ports of call."
"the courses vary—you don’t have to be a computer ninja to apply"