The expense sheet entry can have the project list limited similar to timesheets by using project permission. You can choose setup the option to use project permissions on the project card | permissions fast tab. If you choose Limit Time Entry to Project Permissions set to TRUE. Then setup permissions (Project | Permissions), add the appropriate resource and check 'Timesheet'. Although it says 'timesheet' the settings also apply to expenses.
Julia,
The expense sheet entry can have the project list limited similar to timesheets by using project permission. You can choose setup the option to use project permissions on the project card | permissions fast tab. If you choose Limit Time Entry to Project Permissions set to TRUE. Then setup permissions (Project | Permissions), add the appropriate resource and check 'Timesheet'. Although it says 'timesheet' the settings also apply to expenses.