Navigate to "Public Groups & Posts" by clicking on the icon with three vertical lines in the upper left corner and then click on the world icon on the left panel to access the Public Map. To find groups within a specified region, use the search bar in the upper left corner and enter the desired location. Click on the Menu icon with three vertical in the upper right corner to extend the Map Menu, then "Groups" to explore the Groups in that region. You can also filter your search by keywords in the area that says "Filter by topics or keywords".
Click on a Group to join the group by requesting access, if they are a restricted group, or clicking the "Join [Group Name]" button for a public group.
Hylo will be releasing the Group Explorer feature soon, which will be a directory for public groups.