There were six of us just describes how many people there were. - There were six of us waiting at the bus stop. There were the six of us adds some specificity to the group. Your referring to a particular group of six people. For example, if six people were together telling somebody about something that they did when they were young they might say something like: - It was the six of us and these two other guys that we don't see anymore. Although it's eight people total your talking about your specific group of six people and two other irrelevant people.

The differences is... there where all the six people of a single group. And there where just six people of The numerous group of people on the last sentence.

The difference is very subtle. When you say “There were just six of us” you are merely announcing how many people there were. Then maybe you can start enumerating those people, (i.e “There were just six of us; me, alexander, sofia, ivan...” and so on). However, the main focus on this type of sentence is on ‘us’ and not on ‘who specifically”. “US” can be a nationality, a team, a race, or any other unifying attribute that merges certain group of people along with you. For example: “When I was little I went abroad to study in Britain, and surprisingly on our school there were six of us.” “I didn’t know anyone else knew about the new law, but can you believe it? On the bar I went last night there were six of us who knew”. On the other hand, “There were the six of us” it is implied that the six people are already known by the listener. In other words “the six of us” are individuals and not a group as in the previous sentence. To put it simpler, by adding “THE” you are referring to something specifically. For example: A car, can be any car; THE car, is a specific materialization of the concept “car”: The car of my friend, the car that can drive 20 miles, the car that talks, etc. Hope this helps!

Use "the" with any noun when the meaning is specific; for example, when the noun names the only one (or one) of a kind. Adam was the first man (the only 'first man'). New York is the largest city in the United States (only one city can be 'the largest'). We live on the earth (the only Earth we know).