I think you would be better served by a reseller account.
then you could set up totally separate accounts with their own
login/password, control panel, and space/transfer allotments.
before you buy, ask the reseller provider if they can accommodate your
addon domains on a shared hosting account are very handy, but best used when only one person is involved. if you let others use the space via addon domains, their heavy use of their addon-domain-website could use up all the space/transfer allotment of the whole package, causing the whole group to be suspended for exceeding the allotment.