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|
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).