This plugin does exactly what the name suggests. It allows you to sync up your balance with your party mates (or vice-versa if you are the party leader).

This plugin requires you to have these plugins:
Vault:
https://www.spigotmc.org/resources/vault.34315/

ADP's Parties:
https://www.spigotmc.org/resources/parties-an-advanced-parties-manager.3709/

as well as any Vault-compatible economy plugin.
I use EssentialsX:
https://www.spigotmc.org/resources/essentialsx.9089/