"Joining the party means I have to share xp."
Ya but most games it's usually something like
Solo: 500 xp/hr / 1 = 500 xp/hr
Group: 3,000 xp/hr / 5 = 600 xp/hr
It's rare that grouping with the same level players will net you less xp/time. The only exception I can think of is raiding, but then the goal isn't grinding xp.