Tap on the Jar and then on Users.
You can then choose Invite Users from your Contacts list or someone nearby (as long as you both have your Bluetooth switched on).
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.