To just the author and mods, unfortunately we cannot support this yet, as we don't have a "post author" pseudo-group. That is a good idea, though.
We did accomplish this via nodebb-plugin-support-forum, you could try this and see if it still works?
Chat Restriction
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Is it possible to enable chat feature for the members of a certain group?
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@hnahli there is no straight forward option to add the restriction. you will have to modify core.
