By "my user" you most likely mean your default admin user, correct? For default admin Gluu puts DN of managers group this user belongs to in the `memberOf` attribute. For user entries you create for your own needs you can use this attribute whatever way you like: you can store there the name of the group, or DN of the group in your backend.
Just one point to stress out: though you can see controls to manage groups in web UI, Gluu doesn't make that much use of those beyond their application in UMA/SCIM. So for SAML/OIDC you shouldn't create some groups in Gluu, put users in them etc, it isn't supposed to work this way. What you need is to either create or reuse some attribute, be it" memberOf", "myCustomGroup", or "allowedResources", or something else, and populate it somehow with values that will make sense to your RP/SP doing authorizations. As you have some "external LDAP" (I suppose it's your backend), you already are doing some mappings for you users in CR perhaps. If it's not enough to generate values you need, you could write CR script which will process attributes from backend before storing them in Gluu