Assign subscriber to mailing list

Go to 'View subscribers' on your dashboard or on your left side navigation.

Select the subscriber you would like to edit by clicking on the blue pen symbol.

You will see now 3 different tabs 'Subscriber', 'Mailing lists' and 'Additional fields'.

Click on 'Mailing lists'.

Select the mailing list(s) by ticking the box(es).

Press 'Save and close'.