Presentations are designed for you to share designs privately with your clients on a white label mini-site. There is no public link to a presentation and each person will be required to have a login and password in order to see your presentation mini-site.
Focus groups, on the other hand, are designed to let you invite many people to vote on their favorite designs from your project. You can include people you know, and even people you don't know personally (such as Twitter followers or customers), to help you pick a design in your project. The focus group link will be private but you can always share it publicly and people won't need to log-in to participate.