You are right. Anfängerinnen is the plural of the word Anfängerin.

This could be either:

Capitalized I is there because of gendering.

It is to emphasize that both Male and Female persons are being addressed.

But normally you would write it like:

Anfänger-/innen

Or which is more likely:

Capitalized I is there to emphasize that they are only looking for female students.

Hard to say without context.

You are right. Anfängerinnen is the plural of the word Anfängerin.

This could be either:

Capitalized I is there because of gendering.

It is to emphasize that both Male and Female persons are being addressed.

As alternative you would write it like:

Anfänger*innen

Anfänger(innen)

Anfänger/-innen [repeated to get a visual "complete" list]

Anfängerinnen und Anfänger (as "ladies first")

Anfänger/ Anfängerinnen

Anfänger (as "generic masculinum")

Or which is less likely:

Capitalized I is there to emphasize that they are only looking for female students.

Hard to say without context.

Edited to encompass Shegit Brahm Answer and sebastian redl comment.

You are right. Anfängerinnen is the plural of the word Anfängerin.

The capitalized I is there because of gendering.

It is to emphasize that both Male and Female persons are being addressed.

Normally you would write it like:

Anfänger-/innen

But it can be shorten. To just AnfängerInnen.

You are right. Anfängerinnen is the plural of the word Anfängerin.

This could be either:

Capitalized I is there because of gendering.

It is to emphasize that both Male and Female persons are being addressed.

But normally you would write it like:

Anfänger-/innen

Or which is more likely:

Capitalized I is there to emphasize that they are only looking for female students.

Hard to say without context.