Yes, you might be able to play with your friends but you are no real help for them. More of the opposite (at least in harder content). So if you get the booster refunded (I doubt it): For the love of [insert favorite human god here] stay away from the level 80 booster and learn the game first.
Option 2 (Boost first char): Learn everything the hard way, after being instantly at 80. If you then later want to play the lvl 80 content on another class, you have to manually level the second char, to unlock the content you can do with the first one.
In general, exploration is the name of the game (well, actually its Guild Wars 2 but you know what I mean :P) - whatever you do, it'll likely give you experience, so just do what you enjoy. You'll get all skills and traits by level 80 anyway, so pick what sounds fun and experiment, you cant damage your character in the long run and you can
So you could use the boost on a character who is already level 80 to get it out the shared slot and get the items, then if you want an instant level 80 sometime later use tomes or scrolls to level them up. They already use the "type to delete" feature for items. So why not just use that? 1.
1. It is entirely up to you how you progress. You won't earn any less experience staying in the lower level zones- in fact, you could go clear the other race's starting zones and earn exp at the exact same pace. Fully exploring the map has its advantages- you can earn a bunch of exp, transmutation charges for customizing your appearance or
I should have been clearer in my question. Although the boost is valued at 2k gems, I do not value them very much at all. As mentioned, it is incredibly easy to reach max level. Tomes of Knowledge are easy to obtain for instant boost. Or fast leveling through crafting. In additions, the new Ad N8ggo8C. 316 45 347 203 62 416 174 278 321

guild wars 2 what to do after level 80 boost