Hi!
To claim things you can do /p create <name> then /p claim
Doing /p claim will claim the chunk you are standing in. A chunk is 16 x 16 blocks, do f3 + g to see chunk borders.
You can definately leave items in chests as long as they are claimed.
Yes - Only your party members can access it. and yourself
If any of your party members steals anything, staff will be able to replace the items and punish the thief!