Copying an article with author's name and publishing it in other groups

Ideally, original owner's consent is required. However, it is not always the case the original owner takes a serious view of such violations, if the republication is benign and without any profit motive but only for the purpose of dessimination of information.

Whether the author has reserved his/ her copy right of such an article ?

Generally not.

Hence there is no legal dispute/ issue.

If the author's rights are not reserved and if the article is posted in a public domain also if you are not misusing it for monetary gains then there may not be any legal infirmity in it.