The IESG charter document includes a list of liaison positions
that are part of the IESG a
"part of" seems the wrong term but I now see what you mean
your text seemed to ask who appointed them - (not the IESG's problem,
that is done by the "other" body, but could be an IESG problem if the
wrong person gets appointed :-) )
if you are asking 'who says add or remove a liaison' then, like most
things, it works out to an IESG consensus
but its not the sort of thing that I would think would need to be
called out individually - a blanket statement that the IESG works
by consensus unliess otherwise stated should be fine.
I think the question was more "if someone wants an RIR liaison on the IESG,
who decides that?" or "if someone doesn't want the RFC Editor to have a
liaison to the IESG any more, who decides that?"