Other than the default characters you have available to you, you must acquire Crew Members through taking part in their corresponding Crew Episodes. The best way to start this process is to just run through the first few episodes or chapters (at least through the first chapter) to assemble a decent group to start adding to your list, and then use them to help you with later episodes.
Many of the crew episodes are simply conquering territories or defeating a set number of officers before you can access the final stronghold where the named character is waiting to face off against you. Note that this is actually asking you to gather them as crew members, rather than playable characters, so this means you will not be able to meet the 10 ally requirement until breaching Chapter 2. Here is a list of the available Crew Episodes in the order you'll get them (counting to ten):
Do note that some of these have pre-requisites too. While it may not be in the best interest to do all of these immediately as they're opened (as the later ones are rather demanding on level), getting a small enough group together to help you conquer the Main story will help you in the long haul.