Pullo and Vorenus are Octavian's clients. This is why, when he first introduces them to Atia, he introduces them as his "friends" -- that is what patrons called their clients in front of their clients. The patron-client relationship is also why Pullo ends up teaching Octavian swordplay, and why Octavian helps Pullo deal with Vorenus' unknowing problem. It's not genuine friendship, it's a patron-client relationship.