Find it
Tap the gear in the top bar.

Use it
1
Invite someone
Open the gear and pick 

Invite someone. You get your own link; share it, and when someone joins through it we know you brought them. The dialog shows a count (Nobody has joined through your link yet. or how many people did), never who. Use Copy; if your browser can share (most phones, and some desktop browsers), a share button sits next to it.

2
Block someone
Pick 
Blocked accounts to see who you’ve blocked. Blocked people can’t message you or talk to your brain, and they aren’t told. Tap Unblock next to a name to let them back in; an Undo appears for a moment if you change your mind.
3
Replay the tour
Pick 
Replay the tour to walk through the app again. It takes you to My brain and starts from there.
4
Switch day and night
On a desktop, the sun and moon button at the far right of the top bar flips the look.
On a phone there’s no room in the bar, so it’s in the gear menu as 

Switch to night (or Switch to day).
5
Sign out
Pick 
Sign out at the bottom of the menu. You land back on the sign-in screen.
Good to know
Delete brainisn’t in this menu any more. It sits at the bottom of My brain, so it’s hard to hit by accident. See Privacy and deleting your brain.- People who talked to your brain without a name show as
to your brainin the blocked list.