That seems the most common way to say it by far to me, namlan.
I suppose you could also say:
"They do philanthropic work monthly" but it has a much grander sound to it. Working in a skid row food kitchen is "charity work", while Bill Gates donating $5 million to build a new homeless facility is "philanthropic work", in my understanding of the connotations. While they are both philanthropic work in the strict sense, the phrase "philanthropic work" seems to be applied mainly to large gestures and great works.
You could get more specific and say what they do exactly:
They collect donations for the poor monthly.
They gather used clothing and distribute it to the poor monthly.
They work at fundraisers for charity organizations monthly.