The shortcomings of a small party pale into insignificance when set against the great benefits it will bring to us—time-saving and cost-reducing.

【翻译】相对于带给我们的巨大好处——节省时间和降低成本而言，一个小型聚会的缺点就变得很微不足道了。

【分析】先解决你的两个问题：benifits是名词“好处”，it 指party，it will bring to us 是定语从句，修饰benifits，所以，不需要逗号。

1. 主句：The shortcomings of a small party pale into insignificance，意为“一个小型聚会的缺点就变得很微不足道了”。 pale into“变得”，意思等于turn into。

2. 时间状语从句：when set against the great benefits it will bring to us—time-saving and cost-reducing。省略了主语和be动词，补充完整为：

when they are set against the great benefits it will bring to us—time-saving and cost-reducing.

they 指shortcomings.

be set against：与......相比，与......相权衡。

time-saving and cost-reducing：是benefits 的同位语。

【直译】当它们跟带给我们的巨大好处相比时——即节省时间和降低成本，一个小型聚会的缺点就变得很微不足道了。