Buttons in the Email Design Center allow for creating links that are visually interesting, and distinct from linked images or text in an email. Because they are visually distinct, buttons are great as a call to action - "RSVP" or "Click here to register".
To create a button, select Button Editor from the toolbar.
Click to edit the text of the button. Buttons have the [INVITE_LINK] Dynamic Element - for Invitation Access Types - linked by default, but can be replaced with any URL.
Buttons can have borders, with colors, widths, and styles. Border Rounding makes the button corners more square or more round, and Height Padding and Width Padding make the button taller and wider. The text color, and the button color, are also adjustable.
To edit a button already in the email design, select the button > Button Editor. To change the URL for the button, select the button > Edit Link.
NOTE: Most versions, including recent versions, of Outlook for PC do not render the HTML code for buttons properly. While only a small percentage of overall emails are read using Outlook for PC, if you expect many of your guests to be using Outlook for PC, you'll likely want to use a standard link, or a hyperlinked image, rather than a button.