I've been able to do this via groupmanager's inheritance.
ex:
>you have groups A1 A2 A3 A4, A4 is the lowest rank and A1 is the highest rank
>A4 has the least privileges/permissions, A3 has everything A4 has and more, A2 has everything A3 has and more, and so on
>now you have B1 B2 B3 B4, same thing
>in the groups file A1 does not have B1 (or lower) directly inherited, it may have the same permissions like essentials.home but it doesn't have B1 specified
basically it doesn't have this going on, where B1 is under inheritance.

>now say A3 and B3 have groupmanager.manuadd (so they can use /manuadd and move players to different groups) remember, this also means every rank above A3/B3 will also be able to use /manuadd
if you aren't confused yet... GG..moving along>With it setup this way, people will only be able to move players between groups they inherit, so if your king does not inherit a mod rank, he will not be able to move a player to/from that mod rank. (say its mod<king<knight<peasant) a king will be able to move players who are knights/peasants between the two ranks but will not be able to move anyone to a king rank or higher
>here's an example of the posibilties, keep in mind A1/B1 are the highest ranks on the server, and A4/B4 are the lowest ranks
if Player1 (A1) tries to move player2 (A4) to A2, it will work
if Player1 (A3) tries to move player2 (A3) to A4 it will not work
if Player1 (B1) tries to move player2(A4) to A2 it will not work
if Player1 (A2) tries to move player2 (B4) to B3 it will not work
lastly, don't give out "groupmanager.op" because that overrides the inheritance boundaries.
hope this is what you were looking for