lorde In permissions, you may create groups. Groups are.. well groups of users with specified permissions.. for example Start discussions
can be limited to only group AMA_Speakers in tag AMA
Then default users can have permissions to Reply to discussions
.
Perhaps a image will help.
The below shows that only users in group AMA Speakers may create discussions in tag AMA. However, users may create discussions with other tags like normal and reply to discussions created by AMA Speakers
The below was configured on Devflarum. If you're interested in figuring out something that works before starting your site, feel free to use it