You will setup the membership plan like normal however you will need to change a few important details shown below.
Firstly the membership plan cannot be linked up by the member on their own, the lead can purchase the plan online however you will need to link up the member in club. It might be best to have this as a in club membership only
To make this a joint membership plan you will go to the membership plan, then to details and change the max number of members.
If you set the max number of members 2 this will allow one lead member and one person joint on.
quick tip: the payment will only come out the lead members account therefore you may need to have this at extra to change for the joint member also