As Christian mentioned above, rules vary per university, and there are even sometimes different rules within graduate schools in the same university.
What happens in the majority of cases, is that the supervisor of a PhD candidate (or the department) will invite people to become members of the PhD review committee.
In theory (remember, the rules vary from place to place!), anybody can be invited to be a member of a review committee, however in 90% of the cases, only full professors are invited. Generally, 2-3 people are full professors recruited from within the university, and another 2-3 people would be people from external institutions (also normally full professors).
The reason why the rules allow anybody to be invited is because there are people who, despite not having a PhD or Master degree, have had a long career in industry or in a certain field, and their career achievements are recognized as being significant, and very valuable.
Therefore, it would be highly unusual for a postdoc to be invited to become a member of such a committee. You would need to have some highly specialized knowledge (relevant to the PhD candidate's research topic) that could not be found in any other person. In fact, if a professor invited you to be a member, that person would probably have to write some text or report justifying the reason why you were invited (instead of someone else with a longer, established career).
If you are invited, it should be fine to accept the request, but personally I would feel a bit suspicious (ex: why they cannot find a person with more credentials?)...