them is or them are

The technically correct form is "none of them is ...", but you will hear many people say "none of them are ...".

By the way, I think you want this:

A few groups of people tried, but none of them was able to attack the city.

CJ

Yes I need was, you are correct. But Why do I need to put 'A' in front of few? What I mean with the sentence is there were many groups and each groups was consist of many people ( like 5 to 10 people), so I say, out of those many groups, few groups tried to attach the city but out of those none of are really able to attack the city (due to they are defeated when they tried to attack)

adayYes, I need was; you are correct. But why do I need to put 'A' in front of few?

Few means not many. A few means several.

Few groups of people tried but none of them was able to attack the city.
Not many groups of people tried but none of them was able to attack the city.

adayWhy do I need to put 'A' in front of few?

See Yoong Liat's reply.

If you mean "Not very many groups" or "Fewer groups than expected", then leave your sentence without "A".
I don't think that's what you mean, though, so I think you're going to need the "A few" to mean "some" or "several".

CJ