You can add friends by sending players friend requests through their profile page. They will then get a request for friendship in their messages section. If they accept the request, they will be added to your friends list.
You can send friend requests to players you see in the village, players you encounter in team battles, your clan member from the clan’s member list or players on event or arena leaderboards.
By connecting the game to your Facebook profile you can also see your Facebook friends, who play Angry Birds Evolution, on the Friends tab, next to your Friends list.