Parties are groups that can be made with other members. Like towns, a player can only be in one party at a time. Parties also have access to certain things at certain levels. This may include a party chat, party teleport, alliances, item sharing, and xp sharing (the sharing of xp to online/close party members). You can see if a player in your party is near by doing /party and seeing "Near #/(total party members online)"
To create a party, do /party create (name) (password if you want one)
To invite a player to your party, have them do /party join (your party name) (your party password if there is one)
To leave or join a party, do /party leave/join
When you create a party or are in one, doing /party will give you the stats of your party. The level maxes out at 1000, but you should have all perks by then. It will also list those online in white, the Leader in Gold, and the offline players in grey.
If you have any more questions, ask away or refer to the Party McMMO wiki here.