'Us students' - Does this apposition need a comma?

This is not actually a case of apposition. The phrase us students is a single noun phase and doesn't consist of two adjacent ones.

Basic noun phrases have two main sections, a Determiner and a Head. Typically a Determiner will be a determinative (a word like the, a, some, any). The Head of the noun phrase will usually be a smaller phrase built around a noun. In some grammars this type of phrase is called a nominal *. In the following example, the Determiner is these, and the Head is slimy dinosaurs:

• These slimy dinosaurs

Notice that I said that Determiners are usually determinatives, and the Heads of noun phrases are usually built around a noun. This is because other types of word or phrase can do the job of Determiner in a noun phrase. For example, a Determiner could consist of a genitive noun phrase:

• [That man]'s slimy dinosaurs.

Here we see the noun phrase that man's occurring as a Determiner.

Now, normally, when we us a pronoun, it replaces a whole noun phrase:

• The old bank is being knocked down.
• It is being knocked down.

In the example above the pronoun it replaces the whole noun phrase the old bank. Because pronouns work like this, we normally consider pronouns themselves to be noun phrases, and usually expect them to replace a whole phrase. However, in the same way that we saw the noun phrase that man occuring as a Determiner in that man's slimy dinosaurs, we sometimes see pronouns occurring in Determiner function too:

• you bastards
• we band of brothers
• us teachers
• them bastards (non-standard)

Noun phrases like these are quite rare, but they are perfectly grammatical.

The Original Poster's example

A few of us students have participated in the match.

In the example above, the object of the preposition of is the noun phrase us students. This phrase has the pronoun us as Determiner and the noun students as the Head. This is not a case of apposition. Notice that if we don't use a Determiner here, the sentence will be ungrammatical:

• A few of students have participated in the match.

^{* For example The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002).}