In my shul someone usually brings around the pushka (charity box) during the chazzan's repetition of shmoneh esrei. I have also heard that it is appropriate to give tzedaka during v'yivarech Dovid. However there are people who come into shul to collect at all times of davening (known as meshulachim). Is it permissible for them to collect charity at any time during davening?
Note that a very similar question was asked here but from what I can see seems to only be asking about giving tzedakah, not the permissibility of the asking for it.