Yes, you can group a lvl 1 character with a lvl 80 and both will get XP. But it will be proportional to the lvl, dont expect to kill a lvl 80 monster and give the lvl 1 100k xp.
Also remember that if 2 characters in the party are 10 lvls apart, you wont get any drop or adena. Well, you get Nectars i think...