I haven’t tested it, but did you look at Damselfly? The documentation seems to suggest you can do it: https://github.com/Webreaper/Damselfly/blob/master/docs/Multi-user.md

https://damselfly.info/

Hard to argue with Intel, but I run one of the asrock j3455 boards (with a full PCIe slot and 2 SATA ports) and powershell is reporting OSTotalVisibleMemorySize of 12228504.

Pretty sure I am running a j3455 with 12GB.

edit> Confirmed

The 5 users will only talk to each other, or you have to connect out to the regular phone network? Group calls? Messaging? And when you say secure, do you mean end to end encrypted?