If the server admin has allowed space admins to create accounts then you will see an 'Account Creation' option, otherwise this link will not appear. If you do have this option then you can create new accounts in two ways:
Manually add individual accounts
Upload a tab delimited file containing account details. Information about the format required for this file can be found on the bulk upload page
If you add accounts from within the space admin section, then these accounts automatically become members of the space.