Where in source i can find to set party to where exp is gained without a leech AND when link attack works for everyone and doesnt need to be a full party?
The method CMover::AddExperienceParty is what you're looking for for the party exp share formulas.
As for link attack, you don't need a full party for it to work, by default it's +10% final damage for every member targetting the same mob (or just every group member if you have a scroll of party skill range). Though if you want to change that, then you're looking for CMover::CalcLinkAttackDamage