"Get along well (with somebody)" means you are friendly to each other. "Hang around (with somebody)" means you actually spend time together. You are physically in each other's presence. I can say that I get along well with some people on italki, but because we are in different countries, we can't possibly hang around together.

They are completely different. 'get along with' means that you enjoy each someone's company and have fun together. 'hang around with' means to spend time with friends relaxing or having fun. You generally get on well with the people who you hang out with. In your sentence although the people get on well they wouldn't hang out with other unless Robert is there also.

"Get along well" means that they work well together, or that the don't fight with each other or have any anger between them and they have fun together as friends. "Hang around with" just means that they are together at the same place. Your sentence means that even though they get along well, they don't get together, it hang around each other, unless Robert is there (acting somewhat like a bond between the two)