In Small Improvements the current direct manager is ultimately responsible for the performance review. The direct manager has the ability to share the review and to sign it. Indirect managers can always contribute as well. We additionally offer a feature called secondary reviewers, which allows you to appoint one secondary reviewer for a person as well provided the feature is enabled.
The secondary reviewer can access the performance review and provide insight directly into the same form. However, the secondary reviewer is not allowed to share/hide the review, nor sign it. It's also not currently possible to allow more than one additional person to contribute to a performance review. If this is necessary, the main manager should collect additional feedback by email, and incorporate it himself. Learn more about secondary reviewers in the administration manual.
If you want to collect feedback from several people for several people, then our 360 degree reviews are the way to go. In a 360 review, a person can pick any number of reviewers (or HR or their manager can do it for them). These reviewers all provide their feedback, which is then delivered either to the recipient directly or goes via his manager.
360s don't even have to be anonymous--you can for instance customize your 360s to keep feedback hidden from everyone but the reviewee's manager. In this case an individual reviewer would only see what he wrote himself, but his review would also be visible to the manager of the person he wrote about. This manager would also be able to see who the feedback was from and take it into account during the more traditional performance review. Small Improvements is very flexible, and we're happy to provide guidance.