Tap on the Jar and then on Share. You can then choose someone nearby (you need to activate your Bluetooth and they need to shake their phone) or from your Contacts list. To share the Jar, the person will need to have a HyperJar account. You can then choose what sharing permissions you give them, from paying-in only, spending up to a set limit, seeing all notes and transactions and inviting others.
If you are invited to share a Merchant Jar, make sure you are happy with the permissions you are given before you put any money in.