There are advantages and drawbacks to parties, in particular they affect drops and XP. Every player after the first contributes a bonus 20% to drop rates & 16% to XP rates and the final number is then split across party members.

Number of Players Total Drop Rate Effective Drop rate Total XP Rate Effective XP rate
1 100% 100% 100% 100%
2 120% 60% 116% 58%
3 140% 47% 132% 44%
4 160% 40% 148% 37%
5 180% 36% 164% 33%

One should take into concideration Need-Greed system (used for most of the drops; a player can chose if he/she wants the item and click "need" to roll for it, and the chance of getting the item spreads between other "needers", if any; or just between "greeders" if nobody clicked "need").

Making an ordinary loot drop quicker for one person (newcomer) is more effective in a group of five (e.g. lightning dart). However, getting a loot for one person (newcomer) which is not in the Need-Greed tables might be time-wise to be done in two (e.g. lavender).