You can claim entire chunks under your name, which prevents others from accessing your inventory, breaking blocks, or placing blocks within your area. Chunks are 16 by 16 block areas that extend from bedrock to the sky.
First, you might want to create a party, which is like a list of friends. You can assign parties to a claimed chunk and then later add new members to that party.
To make a party,
Members you add can also add and remove members, but you can prevent this by adding "-m" to the command when adding: /party add -m party_name username. If you've already added the member, remove the member first (with /party rememove party_name username).
Claiming consists of (1) selecting an area and (2) asking the server to claim your area.
Your first 25 or so chunks are completely free. If you need more than chunks than that, you may need to pay something like coal (it depends on what server you play on).
To unclaim an area, select an area first (see above), and then run /unclaim in chat.
If you want to change the party of an existing claim, select the area first (see above), and then re-claim it with the claim command.