First of all, make sure you're logged in and you're the administrator of the canvas:
Click the Admin button in the top left corner:
As an administrator, you can manage multiple aspects of the drawing:
Control each user’s permissions (drawing, adding/removing layers, etc)
Remove users from a board
Set passwords (Pro and Studio plans only)
Eject users from layers
The level of permissions granted determines the level of access of the invited person to the canvas.