All sign in questions

1. I can't login and I'm seeing an error that my email isn't recognized when I go to Launchpad. Why can't it find my account?
2. One of my users forgot their password. How can I get them logged in?
3. I have multiple 37signals IDs. Can I merge them together?
4. How do I get a 37signals ID?
5. I already have a 37signals ID. Do I have to create a new one for a new account?
6. What is a 37signals ID and why do I need it?
7. My Basecamp account is appearing in Launchpad, but now I want to add my Backpack account. How do I add this to Launchpad?
8. How do you log in?
9. How do I link my account with third-party products or RSS feed readers that integrate with 37signals products?
10. The username I want is already taken. How can that be?
11. I'm only a user on one account. Do I still need a 37signals ID?
12. I'm the owner or admin for my account. Why can't I update my user's personal information or their photo?
13. I keep getting a notice that I have to change my username. Why am I being told to do this?
14. How do I enable two-factor authentication for logging in?
15. How do I turn off two-factor authentication?