Recently, I was having a chat with a few senior PhD students (almost done with their thesis) and they pointed out that the professors would, at times, solicit their opinion regarding the admission of a particular student. This was especially true when the prospective student belongs to the same country as that of the senior PhD student.
Does this happen in all institutes?
If it does happen, is it official or discussed-over-coffee unofficial?
Not a particularity helpful answer, but ...
No, it does not happen at all institutes, but it does happen at many. In some cases it is official and in other cases it is unofficial.
As an applicant you should assume that the entire time you are on campus is part of the interview including any drinks/meals.
My department's graduate admissions committee officially includes senior graduate students. And yes, we specifically ask student committee members for opinions about applicants from their home countries. We sometimes also solicit unofficial "over-coffee" opinions if a student knows an applicant (or letter writer!) personally but isn't on the committee.
In our department, we employ the help of grad students (through the grad student committee) to assist with rating institutions (especially in countries that have seen a proliferation of colleges in recent years). We don't usually let students read things like reference letters, since that gets into confidentiality issues.
If the applicant knows people at the department, then we often get informal feedback.
Graduate students will almost certainly not be part of the official committee. However, as Daniel stated, they will may be asked about their feelings about a given applicant to the graduate program.
When applying to join a lab, it is very common for individual lab professors to consult the postdocs/graduate students in the lab regarding candidates.
