iRule for bandwidth throttling per client IP

Are you sure you'll need an iRule? If the same bandwidth limits apply to all clients going to a specific Virtual Server, you could configure one rate class profile and add this to the virtual server.

If you want to define groups and multiple rate class profiles, you could use this iRule below (found here: https://devcentral.f5.com/questions/iRules-for-rate-shaping).

 Paying users get the bandwidth
 User's IPs defined in the iRules/DataGroups

when CLIENT_ACCEPTED {
    if { [[IP::client_addr] eq matchclass gold_users] } {
        rateclass rateshape_10mb
    }
    elseif { [[IP::client_addr] eq matchclass silver_users] } {
        rateclass rateshape_512k
    }
    elseif { [[IP::client_addr] eq matchclass lump_o_coal_users] } {
        rateclass rateshape_128k
    }
    else {
        discard
    }
}