The amount of paint you will use depends both on the size of the painting and your personal style and technique. The amount of paint you use may be more or less than another person in your group.
Others have asked this specific question, e.g. "Amount of paint used in a typical Bob Ross painting"
As you can see by the range of answers, again, there is no mathematical way to calculate how many tubes/grams of paint you will need.
Once you have painted for a while, you will know approximately how much paint you typically use. In the mean time, I suggest visiting a local art supply store if there is one in your area, and ask for their advice. You may be able to buy more paint than you think you will use and return unopened tubes. But we sure to ask about returns before you purchase.
This sounds like a great group activity and a lot of fun.