member groups

For one project I want to allow members to “invite” other members with privilegues to admin their posts – such new members will have their own username and password, but they will be able to manage (change/delete) the original member articles (like admin), but without possibility to change the articles of anyone else. So that the ownership of the article will not be to the one member, but to the self-managed group of members. Is there any way how to do it? Is there any such plugin?
The best would be, if any member could be in any number of “user groups”, and if there would be two roles of each member in any group – “group member” and “group admin” – e.g. only “group admin” would be privilegued to invite other people to the group, only they will be able to delete posts, and they could end membership of anyone in the group (even other admins). The group should have also some group parameters (e.g. name of group).