sometimes people are killing things above there lvl, is easier if u have smn healing u; or just farming, is easier in a party.
Also, sadly: EU seems to end up in NA servers ( no sure why / what the issue was) so some cant ask, and the auto translate (which i discover recently) imho is quite hard ( no that is an excuse, but..)

i had this char invite me to his party to heal him while he killed a monster lvl 31 for a quest (we lvl 29)
him: "together? u heal me? i attack X?
me: we are a lil low and area c 31, but give it a shoot
him: sorry, no? no eng
me: <invite to party>

some might be more shine about their eng or no english.. or like u hard to type

i always ask, but i dont mind the blind unvite (friends invite is a whole different thing) i can leave whenever, i might end up making another friend, could give me extra exp/mats... so why not?. and beside karma: " today you, maybe tomorrow me"

but at the end, your preference. just try to see the good side
enjoy.