There are four leagues in party mode: Bronze, Silver, Gold, and Platinum. Which league you are in depends on your level, which increases as you gain XP from playing parties.
Bronze: levels 1-5
Silver: levels 6-25
Gold: levels 26-50
Platinum: levels 51+
You will only face opponents in parties who are in the same league as you, therefore as a beginner you will only match with other beginners and as you move up in the ranks you'll face opponents suited to your level.