One example was when the group wanted to eavesdrop on Mr. Curtains at night. Kate, is extremely athletic, Reynie is good at logic, Sticky can remember everything he reads, and Constance is very poetic and incredibly annoying. Kate wanted to eavesdrop by herself, and although she managed to hear some bits of information by crawling into the culvert, she wasn't the logical one, and forgot to erase the footprints on the beach when she dived into the sea, which, luckily, one of Mr. Curtain's clumsy assistant erased. However, the closest call was when she stepped on a piece of licorice, a trap that one of Mr. Curtain's other assistant designed, and when he was checking the bottom of people's shoes, she passed it on to the others, and Constance, the last one to get it and having no one else to pass it to, had no choice but to eat to save Kate from being caught. This shows that you can't accomplish something alone.
Another example of needing to work together was when they faced the Whisperer, the machine that was able to brainsweep them. The Whisperer cannot brainsweep you unless until it soothes you, and it has been able to do that with every student of the institute until Mr. Curtains used the Whisperer to brainsweep Constance. She was very resistant, and blabbered out nonsense when the Whisperer tried to calm her down, which confused the Whisperer, and temporarily disabled it. The reason why this example shows the need for everyone to work together is because Sticky, Reynie, and Kate never liked Constance, and often wanted to exclude her from their plans, because she was obnoxious, made poems that teased the others and complained about everything. This event showed how important Constance is to the Mysterious Benedict Society.
In conclusion, it is very important for a group to work togther because one person doesn't have the skills that the others have. Even if it was three of the four people working together, it's like a puzzle with a missing piece, and trying to exclude Constance from the Mysterious Benedict Society was a good example.