I get no more EXP after Lvl 5
You can only get experience from killing your opponent’s units after level 5. Creeps do not grant EXP past level 5.
You can’t get to level 6 off creeps alone. And if your opponent leaves the game, their units are treated as creeps and only give you EXP up to level 5.
I didn’t know this part. What if it’s a 2v2?
It’s a cap on individual heroes, so it depends on what you mean by 2v2.
For example, if you have a level 4 Blademaster and a level 2 Shadow Hunter and you went creeping, both would get the EXP (I think distance plays a part on who gets more EXP, the hero closest gets more).
If you have a level 5 Blademaster and a level 2 Shadow Hunter and you went creeping, only the Shadow Hunter would get EXP. No matter what distance. The Blademaster just stops gaining any EXP from creeps.
The only way to level is to kill enemy units. Anything from peasants to mountain giants, water elementals to Heroes.
A lvl 5 Blademaster and a lvl 2 shadow hunter will gain less exp for the shadow hunter than if it was just the SH fighting alone though.
The lv5 cap has always been there for Frozen Throne, where as Reign of Chaos doesnt have it.
If you bring multiple heroes into a creep fight, the exp are split amongst them all. Any (nearby) heroes that are lv 5 or higher will still receive the split exp, but it converts to zero, so exp is lost/wasted.
Edit: In a 2v2 if an enemy leaves the game, the units converts into the other player’s control, so they are still in the game (exp). It’s only non-exp when the team (solo or multiple) surrenders/defeat.