As dslr595148 stated, you can't forward the same port to multiple machines. What would happen if the router received a packet on that port? Which machine would it forward to?
You're on the right track assigning each user a different external port, i.e. 1111, 2222 etc.
The trick is to change the "port to forward to" so it's not the default "same as incoming".
You want your for forwarding rules to look like this:
User #1. Source port ANY. Destination port 1111. Target machine: server #1, Port to forward to 22.
User #2. Source port ANY. Destination port 2222. Target machine: server #2, Port to forward to 22..
etc.
Be sure you have a daemon listening on port 22 on each server machine (should already be there).