It depends on how many disks you have. If this is just a couple of them, it is IMHO better to increase RG size and add them into existing RG. If you have many disks (let’s say, starting from 10 available) it may make sense to create new RG. In general it is recommended to have raid group of equal size as far as possible.
It really depends on future plans, whether you plan to add more disks later.
Both is possible online without interruption.