If you're viewing a workspace page that you have the ability to join, you will also see an option along the top of the workspace to join it. 

In the workspaces tab, the list of available workspaces are shown.  These include workspaces that you belong to, but also lists workspaces where the workspace security allows you to access the content.

This workspace is named "Sample Project".  Users would click on the word "Sample Project" or the picnic basket icon, which is defined in the workspace settings, to enter the workspace.

When you move your mouse over the entry, more information about the space is shown.  

This workspace card allows you to quickly access the most common workspace operations and you can click the "Join" button to add it to your "My Workspaces" list in your Dashboard.  When you already joined to a space, the "join" button becomes a button to "Unjoin" the space.

If any actions are not available to you, the option will not be shown.  For instance, only Administrators can access the settings of a workspace, and so if there are no settings available, then you do not have Administrator access to the site.

Note: Be careful when unjoining workspaces.  If the workspace is private, then there will be no method for you to join the workspace again.  Guest accounts are unable to leave any workspace they are joined to.