As audience

[Xinfu]

Students may come to the presentation and show their support for friends. If a teacher asks why a student's name is not on the list of those who have to present in this session, could the student say?

-I come here as audience.
-I come here as an audience.

[Rusty]

An audience is generally a group of people, although there are times when an audience of one is possible. To classify oneself 'as audience' makes sense, but I doubt you'll ever hear it used. 'As an audience' makes sense, but the person would be stating that they'll be the only one in attendance. So, unless the person is certain no else will join, I doubt you'll hear it used.

More common:
I'll be in the audience.
I'll be part of the audience.
I'm part of the audience.

[Xinfu]

Excellent answer. Thank you.