Hi Mats,
> Is it possible to hide Gallery, Map Viewer, Scene Viewer from Portal from users?
You can hide the Scene Viewer in Portal for ArcGIS by editing the config.js file. See this help topic,
Set advanced portal options—Portal for ArcGIS (10.6) | ArcGIS Enterprise
> I want the users to access the maps, apps and layers in their groups
Instead of having your end users access Portal for ArcGIS directly, why don't you create some gallery apps from your groups - then just have them access the gallery app?
Create apps from groups—Portal for ArcGIS | ArcGIS Enterprise
Hope this helps,