To view all of your customers, click Customers in the top menu, then Customers.  To filter them alphabetically, click one of the letters at the top of the screen.

Autotask users: if there are customers missing, you can specify the account types that are synced across by going to Admin > Settings > Plugins > Autotask > Account Settings > Sync Account Types.  Customers are synced from Autotask once an hour.