The singular possessive form is referee's; the plural possessive referees'.
It is a plural noun. It is also a verb in the present tense, third person singular.
A referee that referee's a reserve game
The possessive form of the noun referee is referee's.
The 2nd referee assists the 1st referee.
the judges assist the referee.
a referee jersey
An assist referee should know all about football. He should know as much as the referee so that if the referee is blinded or very far away the assistanat referee could give decession after consulting with the referee.
Has been is correct, since we are talking about the present. We wouldn't use have been since that would go with referees, plural, not referee singular. To use had been, we would be talking about something that happened in the past. For example, last year no referee had been sent a request.