In the Source section on the sidebar you can select a specific file in the repository and see who has changed it.

Select a file revision from the History menu and click the Contributors to open a drop-down list of all contributors who changed this file up to this revision.

In the list you can see contributor name, number of lines they changed in this file and number of their commits that modified the file. If a contributor is a Stash user, clicking their name will take you to contributor's Contributions graph where you can check contributor's overall performance and all their commits and pull-requests made in the last year.

To see all repository contributors, scroll the list down to the end and click View all repository contributors.

Stash also can show which lines were changed by a contributor. For that, click Blame and see the change log in the appeared sidebar on the left.