When on a contact record, there is a button at the bottom of the page that allows you to add the contact to a list.
When viewing a list of contacts on a page, you can select the checkboxes on the left side and push button at the top that says Add to List.
But the easiest way to add a large group of contacts to a list is during the import process because on the page where you map your columns on your imported file with the fields in the CRM, there is a drop down button that allows you to add all of the contacts that you import to a list.
If you want to add contacts to a list but you have already completed the import process, it might be best to just import the file again and select the add to list option to get them on the list.
Although, only do this if the file you are importing has email addresses because the system uses the email address field to prevent duplicates because there can only be one contact for each email address. With that, you can import a list multiple times without creating duplicates. But if your file does not have email addresses, when you import the second time to add to the list, there will be duplicate contacts for every record that you import.