Are you sure about that? I don't have the data to argue extremely confidently, but I'm pretty sure that the way I've seen this work it was limiting the bandwidth for the entire group. However, this may be because I applied the group policy at the VLAN level in the MX - not to an individual client/SSID.
If you apply it at the VLAN level then it applies to all the traffic going through that VLAN. So if there was 50 people on that VLAN they would share the 5Mb/s. If you apply the group policy to each individual device then it is per device,