Using other classes as variables

I know this might be java stuff, but Bukkit seems like the best place to go

I'm making a parties plugin for a client, and I'm a bit confused with other classes and using things inside them.

I have a command executor and a class called Party. If you type /party create qwdqwd it makes a new instance of the Party class with the owner and members in.
Here's what I'm confused about, however:
1) How do I update an instance? For example, if I add a player to the party how would I add them to the List<String> in the instance.
2) How would I make the method getParty(Player), where it returns the instance of the class that contains the player in it. Also, where would I have to put it so that an instance of Party isn't required to execute it. I don't want this:

Code:

Party test = new Party(owner, aListOfPlayers);
party.getParty(player);
//I want this instead
Party playersParty = getParty(Player);

 

I have done that, but what would it return? ​

 

@timtower
But that doesn't return the instance of the party, which is what I'm trying to get with the getParty method
public void getParty(Player p) {
for (String party : config.getConfigurationSection("Parties").getKeys(false)) {
if (config.getString("Parties." + party + ".owner").equals(p.getName())) {
return;
}
}
}

 

@timtower
I'm confused. If I make my code

Code:

public Object getParty(Player p) {
        for (String party : config.getConfigurationSection("Parties").getKeys(false)) {
            if (config.getString("Parties." + party + ".owner").equals(p.getName())) {
                return party;
            }
        }
        return p;
    }

Then that just returns my username. I'm trying to make it return the party that the player is in.