Once your club exists, getting players in is quick, whichever way suits you:
Adding one at a time. Just a name is required. Email is optional if you don't plan on giving them a login straight away. Good for a handful of new signings, or filling gaps after a bulk import.
Bulk import via CSV. If you've already got a squad list in a spreadsheet, upload it and map your columns. Name is required, while email and team are both optional. Including a team column lets one file assign players across multiple teams at once. You'll see a preview with any row-level problems flagged before anything is actually saved, so you can fix mistakes before committing.
Don't worry about duplicate-looking names. Dugout allows them without warning, since there's no reliable way to tell a genuine duplicate from two different people who happen to share a name.
Adding a team works the same way at any point, not just during setup: give it a name and it's ready to have players and fixtures attached. There's no limit on how many teams you create beyond your plan's team limit.