I may not understand the requirement either but I think you have 2 problems with what you are trying to do:
** Creating a share through Nautilus doesn't allow that type of thing to happen.
** Even if you did it the old way with a share definition in smb.conf there is no way to differentiate user=multimedia from the guest unless everyone provides credentials which means there are no guests.
My recommendation is to make this so simple that even I can understand it by first going old school with a smb.conf share and then create one writeable share for multimedia and one read only share for everyone else:
*** Go back into Nautilus and "un-share" the folder you shared.
*** Edit /etc/samba/smb.conf and add at the bottom 2 shares - something like this with the correct paths:
*** Unfortunately unlike the Nautilus way to create shares you will have to restart samba after you make the additions:
path = /path/to/shared/folder
valid users = multimedia
read only = no
guest ok = no
path = /path/to/same/shared/folder
read only = yes
guest ok = yes
Then you have to wait a bit for the network to settle down before you try to access the new shares.
sudo service smbd restart
[Share-Admin] will ask for credentials but only allow the user "multimedia" in and it will have write access. The [Share] share will allow anyone in but with read only access.