The way it works is you set the permissions in the UD Settings:
for the access you want to grant to users. Set guest to r/w, r/o, or no access. This will then apply to all UD devices.
You still have to set the credentials when setting up the remote share so it will mount. Then UD puts the appropriate entries in the SMB share config file for the access you have designated.
The SMB config file looks like this:
[XXXXXXXX_Work]
path = /mnt/remotes/XXXXXXXX_Work
browseable = yes
valid users = admin, home, guest
write list = admin, guest
read list = home
case sensitive = auto
preserve case = yes
short preserve case = yes
You'll see that the valid users are specified and the read and write users specified. I set guest to r/w before I captured this file.