Redmine

Dear community,

I would like to have Watchers/Observer outside project teams to be able to see the content of an issue and be notified when an evolution is done to the issues.

Based on my research I have found several plugins:

But It does not seem to give me the desired feature.
Any recommendation for plugin or parameter to enable this?

Best regards.
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I am using Redmine under the following environment but will update soon.
Environment:
Redmine version 4.2.1.stable
Database adapter Mysq