sum of two largest charge group is enormous and bigger than rlist
grompp warns if a charge group is larger than rlist, but if it's crazy large then that might because of a PBC split, particularly if the group has few particles. If so, then a better warning would suggest that the solution is to make molecules whole. In fact, grompp could probably just do the latter routinely after building the topology, which might help for other things.
But if the warning can only happen with the group scheme, then let's not bother.
Remove PBC before generating TPR with group scheme
Ensure that all molecules have been made whole before generating the
run input file when using the group scheme, to avoid error
messages for large charge groups when molecules are broken
over PBC boundaries.