You might need to put all users in one secondary group and change recursive ownership of /tmp to root:that_group. If tmp is system created and locked, you could use a new folder for sharing.
Is that so complicated? But this situation is common, and existing almost every where? Why samba is not using file creation mask 755 specified in smb.conf?
Warm Regards, Mukund Deshmukh, Beta Computronics Pvt Ltd, 10/1 IT Park, Parsodi, Nagpur -440022.