You can create a community for the students, but they have to create their own accounts. Now, you could have them each create their account with the same password, but I don't think that's what you are looking for--is it?
Yesterday I try to give a mome to the community but it does not allow me. but It says: "Web address cannot include a '.' character "...why? i don't understand...the name of the community is "arianna 2.0"
When you create your community, Ning asks :
1- first for the name you want to add to the URL (address) of your community. It MUST BE **arianna20**, such as classroom20, library20. So the url (address) will be: http://arianna20.ning.com/
2- later for the Name of the Community, you can call it arianna 2.0