you will get less exp in party, even if you kill faster it won't work.
it wont be shown as a debuff, but the exp would be about 1/3 missing if you play with 2 ppl in party, 3 and above the exp is/was about half missing.
i tried with my gf to play some chars from lvl 4 to 55 (inkarnations back then in the "old" times

)
and we havent enough exp for the lvl ups in the maps, each time we had to kill/grind additional monsters to get 1-2 lvls for the next map, latest you can "feel" the missing ep at Coba (around lvl 31-34).
we played 2 new chars from 4 upwards again, this time without pary, and no grinding was neccesary.
we tested afterwards again, and have seend its about the ep you will get from killing the mobs (-1/3 to -1/2), the ep from quests stays the same.
so if your ok with grinding or using exp scrolls it's no problem to lvl in party, else i would recommend to lvl without group but running side by side.
you have to kill some "single" mobs twice, and in dungeons/instances you can group up and split afterwards.
That where our experience with parties at lvling, but that was before 5.0, maybe they "fixed" it.
and tutorial zones are no problem if you kill the same amount of mobs at the start, only if one is faster, they have to wait for the other.