Can a user see only their own or their department's items in reports?

Short answer: Not dynamically. A report shows data from the whole process. Create a filtered report for each group and share it with that group, and don't let those users create their own reports.

Why it happens

• Anyone who can create reports in a process can report on all its items.
• A report can't filter by whoever is viewing it.

How to fix it

1. Turn off report creation for these users. Report creation is optional for members and must be granted by a Process Admin. See managing process members and permissions.
2. Create the report yourself and add a saved advanced filter, for example Department = Sales. Members see the reports they created and the reports shared with them.
3. Share the report with that department's group.
4. Repeat for each group.

For items a user took part in, send them to More > Participated items instead of a report. See member actions in a process.

Across processes, an Analytics dataview on the ActivityInstance table, filtered by who acted, shows what a given user worked on.