How do I switch between accounts or add another?

Go to Accounts. It lists every account you created or were given access to; Switch to this account moves you into it. The account chip on the dashboard also lets you switch from a dropdown. Add new account creates a fully separate account with its own channels, websites, people, and billing.

If you were only given access to other people’s accounts, you have no account of your own yet. Add new account then makes your own account, with you as its owner, and takes you into it.