To join tables select the 'Add a join' button. Now, select a dataset to join. If you want to preview the columns in it, click on 'Preview Columns'.
Next click 'Join Table' and select the source, destination and type of join (inner, outer, left or right) if required.
A summary of how tables are joined is provided in the 'Joins' sidebar menu.
Select the 'Verify custom view' button to create a view of the joined tables. If need be, you can preview the data and edit the SQL from here. Once you are ready, click 'Create custom view' to name and save your newly joined table. Once saved, it will be ready to view in the Datasets tab.