Membership into MLSs allows you access to door lockboxes though your Supra Key on your phone.
MLSs often have agreements with other MLSs to allow your Supra Key to access those lockboxes as well.
Below is a chart of MLS reciprocal access.
If you have membership to the MGMLS, your Supra Key is automatically co-oped (your key will open those lockboxes) for CGMLS, GAMLS, and FMLS lockboxes.
GAMLS and FMLS do NOT have an automatic co-op agreement with CGMLS. If you are a member of CGMLS, you may need to call GAMLS and/or FMLS to have your Supra Key Reciprocally programmed.