Is there a multiband compressor plugin available that I can put on a subgroup (for example) to get certain frequency peaks under control when I have a group singing together?
Are you asking which plugin, or which frequencies? Generally speaking, the MultiBD multiband compressor handles broad areas and the DynEQ4 dynamic EQ is more targeted.
A&H also sells bundles of these plugins for the SQ series. You can get the “Dynamics” bundle that includes both the DynEQ and MultiBD as well as a de-esser for $149 which is less than buying the DynEQ and MultiBD separately.