The stamina system is really a non-factor, and something they use to encourage partying. Basically, you have 100 stamina, and once you use all of you, your experience/item rates are dropped dramatically. This resets once per day at 6:00am server time.
You lose 1 stamina per battle you enter. If you're in a party of two, you regain 1 stamina every two battles. If in a party of 3, you regain 2 stamina every 3 battles. You can also recover 3 stamina for every win in the Free League (organized PVP).
Unless you play extremely hardcore, or farm for 5-6 hours straight, you will not run out of stamina. Even while solo. A group of 3 makes it a near impossible task. I've played 4 different mains to level 40+, and have yet to run out of stamina while doing so solo.