You can bulk set a user's project permissions by going to the 'People' section at the top of the screen, and clicking the relevant user. Once you are looking at the user's details page, click the 'Options' button at the upper right of the screen and choose 'Bulk Set Permissions' from the menu.

Next you will be presented with a window that allows you to set the user's permissions across all projects they have access to.

While editing a user's permissions on one project, there is also the option to copy these permissions to all project projects they have access to.