The _hen-or-hun_ question is something many native speakers do not know the answer to and as you will read below, it is really not so important. However, since you will come across both pronouns, it would be nice if you knew what they are about.
'Them' referring to persons
In Dutch, we use two different pronouns for 'them' referring to persons: One for the indirect object (hun) and one for the direct object (hen).
We can even make things more complicated: When the pronoun is preceded by a preposition, we always use _hen_, even though it is not a direct object (which is never preceded by a preposition).
_Hun_ is always 'correct'
If this is all too complicated, just stick to _hun_ in all cases. This may not be considered 100% correct by some but most Dutch speakers mix up _hun_ and _hen_ any way. Wim wrote an interesting forum post about the historical origins of this -apparently artificial- distinction.
We verwachtten hen. | We expected them.
Ze vroeg hun of ze mee wilden komen. | She asked them whether they wanted to come along.
Ze vroeg aan hen of ze mee wilden komen. | She asked (to) them whether they wanted to come along.
In the first example, the pronoun is a direct object (hen). In the second, it is an indirect object (hun). In the third, it is also an indirect object but preceded by a preposition (hen).
Recall that when the noun is not stressed, we simply use the unmarked pronoun _ze_.
Wij verwachtten ze. | We expected them.
Ze vroeg ze of ze mee wilden komen. | She asked them whether they wanted to come with us.
Ze vroeg aan ze of ze mee wilden komen. | She asked (to) them whether they wanted to come with us.