I am puzzled since I tried sudo usermod -a -G input $user but nothing happens. There is no group input in /etc/group, but sudo addgroup input complains that addgroup: The group 'input' already exists. and a listing of /dev/input lists several devices with group id input
I have a framebuffer application, which requires access to mouse/touchpad events and the easiest way (on other Debian derivates like Q4OS) is to add me (and other users who are supposed to use it) to the group input.
The confusion here is probably because the input group is defined in /lib/group because it is a group provided by the base ostree.
I suspect you could duplicate the input group definition in /etc/group and then add yourself to it.