Expanding your clan can be done in a couple of different ways in Ancestors: Humankind Odyssey. The first is through mating which will produce babies, the second is through finding and helping other outsider primates that you find in the world. What you’ll need to do is find these outsiders while exploring and solve their specific problem.
The problems that you’ll encounter are giving them necessary items like food or water. If you encounter an outsider who is hungry, for example, you’ll want to grab a piece of food and bring it to this character to solve their problem. This will allow them to join your clan and expand your ranks. Since you cannot mate clan members who are also family members, you need to incorporate bringing outsiders into your clan as well as breeding to expand your clan.
It’s usually pretty obvious what an outsider’s problem by looking at them closely and listening to audio queues. Some will look visibly hungry or thirsty, but regardless of the problem you’ll need to analyze it and fix it before you can bring them into your clan.
Increasing your clan size through outsiders does give you the added benefit of increasing the number of clan members which could possibly mate with one another. Once you’ve recruited a new clan member, they will need to go through the grooming ritual to form a couple with another.