That doesn't mean friends can't join you, but they "can't mess" with your story. After the 'origin worlds' it opens to multiplayer with "giant" quest chains.
"That’s the part that people really haven’t seen much of yet,” said SWTOR writing director Daniel Erickson, in an interview with VG247. "They’ve only gotten to see the origin worlds." This is where you character class of choice begins their journey in Star Wars.
"The origin worlds are very story-focused, so a lot of the content – although your friends can come with you and play with you – is still really like ‘You’re Luke and you’re dealing with Yoda.’ And yeah, you guys can team up together and you can go and help him out, but about half the content there is solidly single-player," he explained.
"That proportion goes way down as the game goes forward. Again, it’s not single-player, per se. Your friend can go with you, but he can’t mess" with the story.
"Later we get into the multiplayer stuff, and that’s what people haven’t gotten to see. The world arcs are huge. They are double-digit hours of quests – giant chains, all built for multiplayer, all built for your whole group to do this great story together."
"And that’s where you get the actual story of the war. That’s where you get the conflict between the Republic and the Empire. That’s where you’re trying to take over worlds."
"That’s where you’re changing history," added Erickson. Star Wars: The Old Republic releases exclusively on PC sometime next year, hopefully. It also features a starship for players to nip around the galaxy in with companion NPCs. See the E3 trailer.