Tap on the Jar and then on Invite. You can then choose to Invite sharers from your Contacts list or someone nearby (as long as you both have your Bluetooth switched on) or use a QR code.
To share the Jar, the person will need to have a HyperJar account. You can then choose what sharing permissions you give them, from unlimited permissions to paying-in only or spending up to a set limit.
If you are invited to share a Jar, make sure you are happy with the permissions you are given before you put any money in.