union(group_key, group_id, data, modifiers, callback) {
        const identifiers = {$group_key: group_key, $group_id: group_id};
        this._union({identifiers, data, modifiers, callback})
    }
groups.union(group_key, group_id, data, modifiers, callback)
        ---
        merge value(s) into a list-valued group profile property.

        usage:

        mixpanel.groups.union('company', 'Acme Inc.', {'products': 'anvil'});

        mixpanel.groups.union('company', 'Acme Inc.', {'products': ['anvil'], 'customer segments': ['coyotes']});