What are the FHS compliant mount points?

You make your own mount point directories. If you want to ask why, I can only point to the great answer by Wouter Verhelst.

Internal drives

/mnt is a valid place to make your own if you like, and so is /.

/mnt may have been used for this purpose by some historical installation systems, as well as for removable media (before /media). It's still valid for you to do so, but the system itself is no longer supposed to set up anything in /mnt.

I think it's reasonable to use /mnt if you might make multiple mount points. It makes it easy to see all of them together, and it's known as one of the locations people like to use. Some other people like to use /Volumes - following the OS X system, or /vol. /data is common for a single mount point. /d/ is also used. /disk/ is almost certainly used by some, but may be distracting for storage which is not disk-based.

If you use /mnt, I would also create /mnt/tmp. Then there will still be a convenient directory for temporary mounts, the original use of /mnt which FHS mentions.

Preferred mount points for internal HDDs

It's possible that manually creating mount points under /media is a bad idea on some common systems. Modern Linux OS's will create mount points for removable media automatically, and it's possible the structure they create would conflict, or simply appear inconsistent with your own. You don't say what your system is, but you may be interested in portable guidelines, especially if you're asking about FHS. Note this reasoning is similar to why the FHS says the OS must not populate /mnt.

Mount point for system-wide USB disk

Network filesystems

It is sometimes recommended to mount network filesystems in a dedicated sub-directory e.g. /n/host, /nfs/host or /net/host etc.

For example, if you mount a network filesystem at /host and the network becomes unreachable, ls / may hang when it tries to stat the network filesystem. This could be undesirable and frustrating, at a time when you are already becoming frustrated.

When the FHS talks about what you cannot do, then the you it is talking about is a distribution, not a user or local system administrator.

A unix system has only one filesystem tree. You can therefore mount anything in the local system on any place in the filesystem, and you wouldn't be breaking the FHS in any way; after all, the FHS goes to great lengths to stay out of the way of local system administrators. For internal disks that are in the system at all times, it's perfectly valid to have a mount point of /opt or /ntfs or /scratch or whatnot. Alternatively, if you're running out of disk space and you want to move some stuff off your root filesystem and onto another disk, you can move /var or /usr to a separate hard disk and deal with things that way.

The only thing you should not do is to mount an internal disk on a location that the FHS says distributions can write into (such as /usr, /var, or /lib), and then either not move the relevant data to that location, or use some non-POSIX filesystem for the partition in question. Other than that, almost everything is fair game.

To make a directory accessible to specific users, you would create a group. By convention, groups can have home directories. These are sometimes, but not necessarily, created as /home/$GROUP.

It would be possible to create a directory /home/allusers. You could reserve the group name allusers (i.e. create an empty group) if you like. (User and group names are technically separate namespaces, but they are usually kept consistent. E.g. you would not particularly want to create a normal user "allusers" in this case).

This specific naming scheme does not generalize naturally for multiple filesystems, e.g. one internal disk and one network filesystem.

You are not a software distribution, and the rationales given in the FHS does not apply to you. Following this restriction would introduce an additional constraint. If /home becomes unmountable, it would also be less easy to mount /home/allusers for backup, recovery or troubleshooting purposes.