Why you will enter in a party with your friends to lvl up if solo you receive more XP ?
Because being in a party makes you kill things faster?
Also, keep in mind that there's a party exp/sp bonus depending on how many members you have in party.
If you have 9 members in your party, the mob gives 120% more experience in High Five. Which means, that mob that gave you 100k experience when you were farming solo, will give 220k experience, which then is divided amongst the 9 party members. So each particular mob is worth more experience, the greater your party is.
Edit: here is the full table if you want (this is when using the XP party setting as x1)
Solo kill -> 100% exp/sp
2 members in party -> 110% exp/sp
3 members in party -> 120% exp/sp
4 members in party -> 130% exp/sp
5 members in party -> 140% exp/sp
6 members in party -> 150% exp/sp
7 members in party -> 200% exp/sp
8 members in party -> 210% exp/sp
9 members in party -> 220% exp/sp