There is currently no way to group projects in Site Scan using tags or categories. If you create a new project in Site Scan there is no place where you can specify Tags or Categories.
You can log that as an ArcGIS Idea https://community.esri.com/t5/custom/page/page-id/arcgis-ideas
Hello @JedXie as @GlenEsriUK has indicated there is not a way to group projects but we have developed a way here in Australia with our customers by setting up naming conventions on the project. As an example we use
AGENCY - PROJECT LOCATION - PROJECT NAME
Then inside each project we then name the missions with the following structure
PURPOSE - DATE - SITE
Each of these names then enables us to qickly filyer the groups and missions