I do want remote access for myself when I'm not home, but full access. I've already managed that. I also want to be able to setup accounts that are read only for others to be able to access it, but I don't want them changing anything. I haven't figured out how to make an account read only though. I've never looked into VPN, but my router is capable of it, I'm running DD-WRT.
I think part of the in giving a clear answer to your question is revolving around "it." By "it" are you talking about file access, or access to the FreeNAS Admin Web Interface?
If it's file access (via CIFS) you could: Mount the share from your windows box as root, right click the folder the user will be poking and select Properties, then click on the Security tab. From there, edit the permission for the "Everyone" user, and ensure that the users accessing the files aren't, A) explicitly noted with a different permission and B) are not part of a group that has more permissive ACLs.
If it's read only access to the Web GUI - beats me... I'd wager that it's not possible in FreeNAS; read only access to a admin interface is kinda backwards. I've only heard of systems limiting overall access to admin powers, like Solaris with it's roles and such.
You could, I guess, spin up some FreeNAS VMs for others to poke around the Web GUI - that's about the only acceptable use of a FreeNAS VM; training/learning.