What is a word to describe something that belongs exclusively to or is used only by one person or a group of people?

The word reserved (meaning set aside for specific people in this context) works here.

"... the deterring notion that personal assistants are reserved for high-level executives."

ODO:

reserved adjective

2. kept by special arrangement for some person: a reserved seat.

Peculiar to
perhaps not the most common usage of 'peculiar' but i think it would work in the example.

2.1 peculiar to Belonging exclusively to:

• ‘some languages are peculiar to one region’

• ‘ I'm not sure if all people do this, or whether it's something
  peculiar to my family.’

In your example:

the feature film reinforces the deterring notion that personal assistants are peculiar to high-level executives."

Though I might rephrase to something like

the feature film reinforces the deterring notion that having personal assistants is peculiar to high-level executives."

Alternatively 'prerogative' may be what you are looking for

A right or privilege exclusive to a particular individual or class:
‘in some countries, higher education is predominantly the prerogative of the rich’

Per your example

the feature film reinforces the deterring notion that personal assistants are the prerogative of high-level executives.

This can also be intensified by the use of 'sole'

the feature film reinforces the deterring notion that personal assistants are the sole prerogative of high-level executives.

Depending on your intention, you may wish to consider the word shibboleth.

noun: shibboleth; plural noun: shibboleths

A custom, principle, or belief distinguishing a particular class or group of people, especially a long-standing one regarded as outmoded or no longer important.

You could use this word in your example (though it may be stretching the use a bit) to imply that having an assistant is an implicit show of power or position.

Exclusive (to) works particularly well here:

exclusive adjective

(postpositive) foll by to. limited (to); found only (in):

this model is exclusive to Harrods

As well as meaning something belongs to a particular place, group or individual, it connotes a degree of privilege and status often associated with the group under discussion.

Limited (to) and reserved (for) have similar connotations.

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More generally, idiosyncratic (to) and peculiar (to) mean exactly what you intend:

idiosyncratic adjective

[…] something peculiar to an individual

peculiar adjective

4. belonging characteristically (usually followed by to):

   an expression peculiar to Canadians.

5. belonging exclusively to some person, group, or thing:

   the peculiar properties of a drug.

sole purview

As in "personal assistants are the sole purview of high-level executives".

It's a common and somewhat formal/legal idiom that feels best for this situation.

Here's an example usage -- "Will print books be the sole purview of the wealthy?"

purview

2 : the range or limit of authority, competence, responsibility, concern, or intention

"Purview." Merriam-Webster.com. Merriam-Webster, n.d. Web. 11 Jan. 2017.

Endemic

1. native and/or restricted to a particular area or field
2. characteristic of or prevalent in a particular field, area, or environment

As in personal assistance, belong to, and are inherently restricted to Executives. This also conveys that this is the natural place for PA's, and the area/reason to which originally gave rise to the role.

A lot of possibilities have been mentioned, but the one I don't see so far is:

Dedicated (adjective)—Exclusively allocated to or intended for a particular purpose

from Oxford Dictionaries

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Some possible phrases:

Geared towards

Meant for

Customized to

A little different, but how about fodder?

OD:

fodder: a person or thing regarded only as material for a specific use

Your example:

"... the feature film reinforces the deterring notion that personal assistants are the fodder of high-level executives.

Minion (but also) devotee, without being derogatory, i.e. as minions to top chefs, hairdressers, designers, who would do 'anything', which may seem demeaning or not part of their job description who learn from amongst the best persons in the industry as well as paying their dues.

Minion: a follower or underling of a powerful person, especially a servile or unimportant one (a follower or underling of a powerful person, especially a servile or unimportant one).https://www.google.com/search?client=firefox-b-d&q=define+minions

Property fits your sentence: "...the deterring notion that personal assistants are property of high-level executives."

It simply means (at risk of committing the etymological fallacy, perhaps) "Something belonging to [someone]".

property (n)
c. 1300, properte, "nature, quality, distinctive character always present in an individual or class," later "possession, land or goods owned, things subject to ownership" (early 14c., but this sense is rare before 17c.), from an Anglo-French modification of Old French proprete, "individuality, peculiarity; property" (12c., Modern French propreté) and directly from Latin proprietatem (nominative proprietas) "ownership, a property, propriety, quality," literally "special character" (a loan-translation of Greek idioma), noun of quality from proprius "one's own, special" (see proper) --etymonline

It is commonly understood now that one person can never really be property of another, but many legal systems since the beginning of history have recognized the "owning" of another person. For a rather ironic example, as Paul Finkelman points out,

The American Constitution does not mention slavery until 1865, with the adoption of the Thirteenth Amendment, which abolished the institution. Yet the Constitution, written in 1787, is riddled with provisions tied to slavery which protected it without naming it. [...] A constant theme of this discussion is how the legal system balanced the dual status of slaves as ‘people’ and as ‘property’. --Slavery in the United States