There are many ways to join an official series.


From the iRacing UI (aka desktop icon):


Click Racing...Official


    Under that you can choose either Official Series or Up Next




In Up Next or Current Series, you can Filter to show what you want to see



and in the results, you can sort by any of the columns, for example, License Group



then click the green Register button for the race (or session) you want to get into


Follow the online instructions and then click Join when prompted: