I want to add the Users and Group link in the toolbar, next to Sharing
Did you mean the "Sharing" Tab (its not identical, but targets the same)?
Or, maybe this one helps: https://docs.plone.org/develop/plone/functionality/actions.html
I want a link next to Sharing in toolbar, sorry for not being precise, I have edited the question with the screenshot
like @mgraf said: use the actions. Go to users and groups, copy the URL in the browser link window. Then go to the actions configuration, add new action, name it 'users' or whatever, add to the object action category and click Add. After the new action is created click on the button to modify it and put
String: <copied URL>
in the 'Action URL' field. Click Apply. Job done.
It would make more sense to add it in the User actions since it is not a 'object action' in fact but there is no provision in Plone for a 'custom action you put anywhere you want'.