Setup multiple copies of SM on your server under different dirs. Then each would have their own login and password.
This is exactly what I recommend.
ie:
/home/user/www/snippetmaster1
/home/user/www/snippetmaster2
/home/user/www/snippetmaster3
/home/user/www/snippetmaster4
Each folder above would have its own completely separate installation of SnippetMaster.. so you could set up individual usernames/passwords, settings, etc...
No worries about licensing.. as long as they're all on the same hostname you will be fine.
The next version of SM will have multiple user functionality, multiple snippets per page, etc.. you will be able to specify users for each snippet.. but it will be another month or so until release. Sign up for the mailing list to be notified of new releases as well as to know when I'm looking for beta testers.
Cheers!