**Admittee** describes such a person. 

From [Webster’s][1]:

> :a person who is allowed to enter a place (such as a college or
> university) :a person who is admitted 
>
> But the college can’t know
> :a person who is allowed to enter a place (such as a college or
> university) :a person who is admitted.
> Worse, it can’t know which ones. In putting together its **admittee** list
> it took great pains to strike a balance—men and women, athletes and
> musicians, black and white, rich and poor. —James Fallows,  The
> Atlantic,  November 2003

The most common way I have read colleges and institutions referring such people is simply as “admitted candidates” or “admitted students”.

  [1]: https://www.merriam-webster.com/dictionary/admittee