Registered user only

Hello,
I need some help with this plugin.

How to make this plugin work for registered users only, with out them needing to enter their email they will just have to press a button and they will be added to the contest. Non resisted user will see a button that says register to join the contest.

Any help is appreciated.

hanks

https://wordpress.org/plugins/contestfriend/