There are a few options here, but they will become easier in time (R82.x if I recall) . For now, automated tools that dont allow you full control of what you query may have some difficulty in accessing all the OID offers.
Today, you need to query the device using a specific OID(s). Attached is an example using SNMPv3 that a colleague helped me with. Rename from .txt to .ps1 and it will offer you a powershell script. You need to adjust the top section with your details (the section "WalkRequest").
In R82.x, we will see 2 improvements (planned).
1 .You will be able to access the Security Group using SNMP v2 using the SMO IP address, but, each SGM will be contactable using a unique port offset from a base. For example A.B.C.D:2050 could be SGM 1, A.B.C.D: 2051 would be the second SGM.
2. The other method planned, is to use SNMP v3, and query each host (via the SMO) using a context field. Today, we use this with VSX to query a specific VS, but the plan is that you can do the same with the SGM.