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. How do I set up and use phone verification for logging in?
6. I already have a 37signals ID. Do I have to create a new one for a new account?
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. What is a 37signals ID and why do I need it?
9. How do you log in?
10. The username I want is already taken. How can that be?
11. How do I link my account with third-party products or RSS feed readers that integrate with 37signals products?
12. How do I turn off phone verification?
13. I'm the owner or admin for my account. Why can't I update my user's personal information or their photo?
14. I'm only a user on one account. Do I still need a 37signals ID?
15. I keep getting a notice that I have to change my username. Why am I being told to do this?