set(group_key, group_id, prop, to, modifiers, callback) {
        const identifiers = {$group_key: group_key, $group_id: group_id};
        this._set(prop, to, modifiers, callback, {identifiers});
    }
groups.set(group_key, group_id, prop, to, modifiers, callback)
        ---
        set properties on a group profile

        usage:

            mixpanel.groups.set('company', 'Acme Inc.', '$name', 'Acme Inc.');

            mixpanel.groups.set('company', 'Acme Inc.', {
                'Industry': 'widgets',
                '$name': 'Acme Inc.',
            });