Is it possible to show a resource only to a certain group?

Yes, it is. The access to each resource or activity can be limited to one or more groups of users. First of all, make sure that you have created the groups within your course (see the following FAQ: Can I create groups within my course?). After that, click on Edit besides the resource for which you want to restrict the access, then click on Edit settings. Under Restrict access, click on Add Restriction and choose Group among the listed restrictions. You can choose one or more groups. 

(last update: 24.07.2018)