The IBM System z Remote Development Programs (zRDP) [1] costs a little less than $600 per month, and gives you access to a system sufficiently robust for learning. I don't know how it would work to have multiple IDs for multiple people on the same subscription. But if you can trust everyone in a group of 10-20, and work out a shared access schedule, then it can be quite affordable. You might need to come up with some development project as a stated reason to join, but that is easy enough.

[1] http://dtsc.dfw.ibm.com/MVSDS/%27HTTPD2.ENROL.PUBLIC.SHTML%2...