Is it grammatically wrong to say "People in here"?

No, it's perfectly correct.

If you're in a room or some other enclosed place, you can refer to "people in here".

See examples in published books for "people in here are" http://www.google.co.uk/search?q=%22people+in+here+are%22&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GB:official&client=firefox-a#q=%22people+in+here+are%22&oe=utf-8&rls=org.mozilla:en-GB:official&client=firefox-a&um=1&ie=UTF-8&tbo=u&tbm=bks&source=og&sa=N&hl=en&tab=wp&bav=on.2,or.r_gc.r_pw.&fp=e0d132a3031d2d2f&biw=1152&bih=618

Yes you can say "people in here", as in:

All the people in here are drunk.

no. its not incorrect if they are 'in' a room, 'in' a building... .

but 'people here...' i would think is more 'accepting'.

it is grammatically correct due to regions or different kinds of speakers sometimes even standard user also use people in here sometimes standard people dont say this structure

it is grammatically wrong due to regions