Occupancy seems like the natural word for what you ask, per oxforddictionaries:
the proportion of accommodations occupied or in use
(Edit: The most applicable sense of it in wiktionary is
The act of occupying, the state of being occupied or the state of being an occupant or tenant
which is rather less satisfactory than the definition shown first.)
Here are some other possibilities: population, headcount, crowd size, census, number, complement, company, fraction. A rarely used term is filledness. Of all these, headcount perhaps fits best into your example, but others fit with minor rewording:
The elevator's headcount is six at the moment.
Those in the elevator number six at the moment.
Those in the elevator are six in number.
The elevator party is six in number.
There are six persons in the elevator.
Comment: As the last of those examples illustrates, one doesn't need a special word (besides in) and you may need to revise your question if that being so is unacceptable for some reason.
I believe that in this case it means the person who wrote the letter does not know the proper usage of ibid.. The proper usage of ibid. is in a bibliography or footnote to refer to the previous citation.
From the context, I would assume that the writer of the letter mentions the interview in a prior sentence, and now is trying to recall the same information.
For example:
We will be interviewing you on our show "Brightly Colored Morning Extravaganza."
You are requested to appear for ibid interview on Thursday at 7 AM.
Unfortunately, this seems to be the work of a pseudo-intellectual. I know this because, as the adage goes, it takes one to know one!
Best Answer
No. This is the fourth definition for "capacity" in the Merriam-Webster dictionary:
So, possible answers to "in what capacity" here would be, for example:
I worked with the referee at company XYZ, where we were both software developers.
The referee was my Maths teacher at school.
And an answer to the full question "How long have you known this referee and in what capacity" would be, for example: