Microphone for a vocals with big band

Date Updated: September 1, 2017 FAQ #812
We have a performing big band that has recently brought on a vocal quartet. We are searching for a microphone that the group can gather around and sing, and we don't have a clue what kind of mic would be best for something like that. We are having monitor feedback problems with the ones we've tried so far. What do you suggest?

Consider any vocal mic with a cardioid pattern. This pattern has a pick-up angle of 130 degrees. Position the four singers so all four mouths are within the 130 degree angle.
Models to consider: KSM32; Beta 87C; SM86; SM58.
Also, consider the use of in-ear monitors.