This has been implemented in AMS 2.2.20 (as a favor to @alfa1 )
This is both a permissions driven feature and a per category feature (requires both in order to be used).
I've added a new Article permission "Self join contributors team on any article within categories that allow self join contributors team" as shown below.
I've added a new sub-option to the "Allow contributors and co-authors" per category setting "Allow self join contributors team" (as shown below).
When the viewing user is viewing an article that is in a category that allows members to self join AND the viewing user is not the article author AND the viewing user is not already a contributor (or co-author) AND the viewing user has the permission to self join contributor team, the viewing user will see a [Contribute] button (as shown below)
Clicking on the [Contribute] button will launch a standard confirmation overlay (as shown below).
Clicking on the [Join team] button will add the viewing user to the Article contributors team. As you can see, Mr Tester is now listed in the Article contributors block and has access to perform many of the same tasks as the article Author does (as shown below).