This à is a preposition and is not part of the verb form. This sentence is based on the construction “être [un certain nombre] à” like in:

Ils sont 3 sur 4 à regarder la télé plus de trois heures par jour.
Ils sont plus de la moitié à regretter son départ.

This preposition is then followed by a verb in infinitive form. In French an infinitive can be given an “accomplished aspect” (aspect révolu), which is formed using the appropriate auxiliary verb (it depends on the main verb) and the past participle. In your case “avoir” becomes “avoir eu”. Other examples:

Elle se rappelle avoir marché sur les Champs Élysées.
Il pensait avoir résolu le problème.
Ils croient être allés sur Mars.

And in passive form:

Il croyait avoir été compris.

You can translate it word by word to “to have had”, and as far as I can tell, it has the same meaning.

Avoir eu is called the past infinitive of avoir, and “indicates an action that occurred before the action of the main verb, but only when the subject of both verbs is the same.”

The second part of it is “être quelques-uns à faire quelque chose”, which does not translate very well in English, but means “those who did this (or to whom this happened) were, say, a few”.

Here, “they were 2 out of 14 to pass the exam” seems not so poorly said. (Or “… to have received their diploma”, to stick to the french construction)