I uncommented it and set the rights as you advised:
pi@raspberrypi:~ $ cat /etc/rc.local
#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
#hciconfig hci0 up
#hciconfig hci0 pscan
"/opt/retropie/supplementary/retropie-manager/rpmanager.sh" --start --user pi 2>&1 > /dev/shm/rpmanager.log &
exit 0
Result:
pi@raspberrypi:~ $ /opt/retropie/supplementary/retropie-manager/rpmanager.sh --isrunning
RetroPie-Manager is not running
Sad.....