Your group should be neither too big nor too small. Groups that are too big are difficult to manage, have an impact on the environment, and annoy other walkers. But what's too small? Solo walking is not recommended. However, the reality is that many people walk on their own, myself included. If you are going to do so, you need to understand the risks you are taking, and the consequences if you are injured or become lost. Of course, if you don't leave information about where you are going, this becomes even more risky. The usual suggestion is to walk in a group of three or four. In this way, if one person is injured, one or two people can go for help while one remains with the injured walker. Just be aware of the risks if you don't follow this advice.
Apart from this, the group will dictate how you walk. You need to assess the age, health, fitness, experience and ability of each member. The overall walk will have to be tailored to suit the slowest, least able party member. Unfortunately, sometimes this should mean that some people cannot go on particular walks. Don't try to push yourself or others beyond your/their abilities. Each person in the group should be aware of the walk route, alternatives, risks and rules. Each person looks out for the others around them, and unless the group makes specific arrangements to separate, the group stays together. This last means that each person must maintain contact with the person behind them.