You invite guests via a personalized email message. Guest members have a similar project access compared to yours. Whether the receivers of your projects can make changes to the project or not can be defined from the invitation dialog.
When you invite guest members, they receive an email invite with a link to activate their access to your project. To activate the access the guest needs to follow the link from the invite. Then they can access your project when logged into a Picter account under "shared with me" at any time. 

To activate the access the guest needs to follow the link they receive via email. For the activation, they either have to create a Picter account, or login to their existing one. Once activated a guest can access your project when logged into their Picter account under "shared with me" at any time.