The getmemberroles command returns a CSV list of role names that a group member belongs to.

llInstantMessage(CORRADE,
    wasKeyValueEncode(
        [
            // returns the roles that the
            // avatar named "Johnny Bravo"
            // belongs to for the given group
            "command", "getmemberroles",
            "group", wasURLEscape(GROUP),
            "password", wasURLEscape(PASSWORD),
            "firstname", "Johnny",
            "lastname", "Bravo",
            "callback", wasURLEscape(URL)
        ]
    )
);

The Linden Lab / Second Life grid protocol states that everyone inside a group is in the Everyone role regardless of what other roles they may be in. Similarly, you can only remove someone from the Everyone role by ejecting them from the group.

It would be superfluous for Corrade to tell you that someone is in the Everyone role - because if they are not, then that person is not inside the group in the first place.

The following is a short pseudocode to determine:

• if an agent is in the group
• the roles of that agent

given an issued getmemberroles command.

if(success == False) {
     if(status == 19862) {
        // agent is not in group
        return;
    }
    // some other error occurred with status not 19862
    return;
}

if(success == True) {
    // agent is in group
    if(data is empty or undefined) {
        // agent is only in Everyone role
        return;
    }
    // agent is in the Everyone role and other roles
    return;
}

Note that the correct way to branch on Corrade error messages is to test the status key for the numeric value because the numeric values are and will be unique regardless of the Corrade version whereas the English message agent not in group may change or be localised.