How do I add a constituent to a category?

You can add a constituent to a constituent category by opening their record and selecting the category in the white space under Category.

If you need to add multiple constituents to a category, if you can identify them on a grid you can use the Actions menu > Batch Update feature to add them add to the same category. You can learn more about this feature in this article https://support.donorview.com/en/support/solutions/articles/9000239074-batch-update-records-constituent"Batch Update Records - Constituent."