Sometimes emails get lost and a customer might not have seen the initial invitation email regarding their Subscription or Payment Plan. Here is how to send them a followup.
Currently there is no automated or "one-click" option to send a followup to the initial invitation email, BUT there is a quick workaround to this, even if it isn't optimal.
1. Navigate to the "subscription" or "Plan" you wish to send a reminder to (That has an existing subscription or payment plan) via the Plans & Subscriptions tabs.
2. Located the customer you wish to notify and click on their name.
(This will display all information pertaining to their subscription or plan) 
3. Click on the "Launch Subscription/Plan Signup Page"
(This will open up the same webpage that is embedded on the email invitation sent to the customer initially.
4. Send this to the customer via whatever communication method you utilise (SMS, Email)