Methods for assigning permission sets to one user versus many users
Key Takeaway
To give one user permission sets, start from that user's detail page and click "Edit Assignments" in the "Permission Set Assignments" related list. To give one permission set to many users, start from the permission set, click "Manage Assignments", then click "Add Assignments" and select the users.
You can assign permission sets from two directions. Start from a user to give that one person one or more permission sets. Start from a permission set to give it to many users at once. For large access changes, such as moving a whole team off profile-based access, a manual user access policy can grant a permission set group to every user who meets your criteria in one operation.
Choosing where to start
Pick the method by where you start.
If you need to give one user one or more permission sets...
Start from the user detail page. The user access summary also works for a single user.
If you need to give one permission set to many users...
Start from that permission set's page.
If you are making a large-scale access change, such as moving a whole team off profile-based access...
Use a manual user access policy.
Dana joins the maintenance team and needs three permission sets. You open Dana's user detail page and add all three in one edit. The next week, 25 field technicians need the same new permission set. You open that permission set instead, click Manage Assignments, then Add Assignments, and select all 25 technicians in one pass.