i think there are some words you can say with 안녕 ... 잘가 or 잘가요to your friends when you meet your friends then go , 또 봐 or 다음에 봐 (it contains the meaning.. 'see u next time'.) you can add it to 잘가 and 들어가, (when your friend go home directly) of course, 안녕 is good too and simple and natural.^^

안녕 is saying hello and goodbye to a friend with the same age 안녕히가세요 is saying goodbye to someone who is going somewhere. (take note that there is 가 here which means go) 안녕히계세요 is saying goodbye to someone who is staying (계 means stay) hope this helps

안녕. Say Hello to your friends. 안녕. Say Goodbye to your friends. 안녕하세요. Say Hello to older people. 안녕히가세요. Say Goodbye to older people.