There is no distinguish between someone being adopted or born in the family You can add an adopted person under their adoptive parents and then you can put in their bio they are adopted. Even list their original surname if it's known. If their birthparents are also on the tree then you have a few options:
1. List them under the birthparents and then in the bio list who their adoptive parents are
2. List them under the adoptive parents and then list in the bio the birthparents.
3. You can list them under both the adoptive and birth parents. Yes, that would mean they'd have two boxes on the family tree.