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When I add a user to a new group in Gnome3 and log out of Gnome and login again, the user is still not part of the group. After rebooting the computer the user is in the new group. I suggest that gdm somehow reloads the group information before logging a user in.
I assume this is some sort of NSS problem are you using SSSD? (moving there for now, but it might get bounced around if you're using nscd or something else instead)
Do you use sssd? what is and output of following commands after login id id $current_user
Additionally the content of /etc/nsswitch.conf might be interesting. If e.g. there is passwd: files sss group: files sss but initgroups: files updating group-memberships might not work as expected, in this case just comment out the initgroups line.
FYI: in the meanwhile I upgraded to Fedora 25. How can I determine whether I'm using SSSD or nscd? The output of id and id $current_user is identical: $id uid=10015(vlugja) gid=15002(domain-users) groups=15002(domain-users),10(wheel),100(users),498(dockerroot),500(groupjan),15003(docker) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 id $current_user uid=10015(vlugja) gid=15002(domain-users) groups=15002(domain-users),10(wheel),100(users),498(dockerroot),500(groupjan),15003(docker) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 Excerpt from /etc/nsswitch.conf: passwd: files sss shadow: files sss group: files sss #initgroups: files
Do you have configured sssd? Do you have installed nscd? Could you share /etc/nscd.conf and /etc/sssd/sssd.conf?
I have no access to the affected systems any more, so unfortunately I cannot provide the needed info. Feel free to close this bug if you have insufficient data to work on it.
OK, feel free to reopen in future.