groups of five or more 的意思是“五人及五人以上的团体”。

Groups of five or more require special arrangements and must be confirmed in advance. 的意思是：五人及五人以上的团体需要特别安排的，必须提前确认。

又如：

There are generous discounts to groups of ten or more. 对10人及以上的团体有大幅度的折扣。

We were warned not to walk in the mountains except in groups of three or more. 有人提醒我们，没有三个人三个人以上（即不要低于三个人）不要在山里行走。

大彭老师的解答很好。这里补充几个类似的表达：

The guide took us round the museum in small groups of three or four. 我们三四个人为一个小组，由导游带领四处参观了博物馆。

Groups of six and above [=of six or more] need reservations. 六人及六人以上团体需要预订。

Five women in a group of 18 people is a pathetically small number. 18人的小组中只有5个女性，真是少得可怜。

A group of 80 attackers launched an all-out assault just before dawn. 有80名攻击者的一个小队在黎明前发起了一次全面进攻。