There are two kinds of categories - organization wide categories and group categories. Group categories are uniquely defined within the group and can only be discovered by those with access to the content in the group.
Organization categories can be discovered by anyone with access to the content across the organization and are "outside the box" of group categories..
You can add content to a group by filtering based on organization categories. When you set up categories in the group, they are unique to the group and only apply to content in the group.
This is a dated blog article, but is still valid.
https://www.esri.com/arcgis-blog/products/arcgis-online/administration/content-categories-four-ways-...
I think it would be beneficial and useful to search group content by organizaton categories, or to be able to set up a group using existing organization categories rather than create unique ones within the group.
I'll take this up with the dev team, but encourage you to add this as an idea if you agree and get others to vote it up.