This is a common question asked by HR.
If you're a member of a bargaining unit, then your union steward can accompany you as representation. Otherwise, most companies absolutely do not permit employees to bring witnesses into private meetings such as performance discussions, investigation meetings, etc. Note, however, that the company at its discretion may pull in additional members of HR or management as witnesses to the discussion.
I'd imagine if you're dealing with an inexperienced supervisor or HR representative would you ever get lucky enough to have someone who allows you to bring a buddy.