Hi, there.
Well, I do not use MEL but use Python.
If you don't mind, You can use the Python code below:
import maya.cmds as mc
sCurrentRL = mc.editRenderLayerGlobals(query = True, currentRenderLayer = True)
sSelect_L = mc.ls(sl = 1, l = 1)
mc.editRenderLayerMembers(sCurrentRL, sSelect_L)