Hello

Where is the setting in the ACP where i can set a member group to be shown in the @ feature ?

I mean that when i type the @ there is a certain member group who doesnt show , the feature works fine for other member groups and i dont know why

This is not something available in vanilla IPS.  

You likely have a custom plugin for group mentions.  

As mentioned above, mentioning a group is not supported by default. There is a modification available for it though:

 

With the marketplace closing you can't purchase it anymore, though. It doesn't seem this developer has set up a place somewhere else to sell his mods yet, but try posting in the support topic or contacting him directly.