device or resource busy: /dev/ttyUSB0 when not root

I've got an arduino that I use echo "b" > /dev/ttyUSB0 to send data to it. When this command is run via the root user it works correctly. However a normal user gets an error of:

echo 'b' >  /dev/ttyUSB0 
zsh: device or resource busy: /dev/ttyUSB0

How can I give a regular user permission for this?

ls -altr /dev/ttyUSB0
crw-rw---- 1 root dialout 188, 0 Nov 30 22:40 /dev/ttyUSB0

My user is in the dialout group, so they should have permission.