Click MY CONTACTS on the top of the page
Hover over More Options and select Registration Form
Your registration page will be public, so anybody can register as a contact regardless of how they came across the page. If you want to proceed, toggle the switch to ON.
A custom link will get generated for you. Copy and paste it into emails or your website for prospective contacts. If you want new people to automatically get injected into specific groups, select those groups and use the second generated link instead.
When someone clicks your link, they will get redirected to a screen similar to the one below where they can register to get put into your contact list.
When you are finished accepting new contacts, be sure to come back to the Registration Form screen and toggle the switch to the OFF position.
Read how to import contacts using a CSV file