It can depend on the formality of the situation, but generally something as simple as "hello everyone", "hi everyone", or "good afternoon" will do, especially among friends and other informal situations. In a formal situation, you might want to avoid the "hello" and "hi", and wish the participants e.g. a "good afternoon/evening".

If you don't know the people sitting at the table, it's nice to ask first: "Do you mind if I join you?" Or: "Is this seat free?"