It's a no brainer, that going on vacation with a group is definitely more cost effective compared to just going alone or with another person. Meals, travel, accommodation are costs that can be split evenly, and the more the merrier to keep the costs low.
While it won't work for every vacation you take, there are certain trips you can organize to get a group of people to go with. For example, tours, or beach holidays. And when it comes down to numbers, the larger groups get a better rate at a lot of things.
If you've got a group of ten or more people, ask about a group rate with everything you do. If you're flying, ask your airline about a group rate. If you're staying in a hotel, ask about a group rate. If you're going to a museum or a garden or an exhibit, ask about a group rate.