In discourse is there any api , by which i can assign complete group under a topic. another doubt is while creating a topic we are assigning some user name using comma separated , so is that message is visible to only to those user whose username we added or is it visible to all and all can response.
There are no per topic permissions.
You can @mention a group, or a several users. I don’t know whether the assign plugin will allow multiple people to be assigned, but I doubt it