Answer
This isn't missing so much as not yet built — Solution Manager wasn't fully converted to Aurena on Apps10 at that point, and user administration (along with a number of other Solution Manager configuration areas) still needs to be done through the classic IEE client rather than Aurena. If full Aurena-native user administration is required, that depends on which release/track you're on, since this has continued to be built out over subsequent IFS Cloud releases.