Adding, editing, and removing co-author information is well described topic at RG Help Center page:

https://explore.researchgate.net/display/support/Authorship

Dear Michel Meignan ,

please have a look at my answer from yesterday:

https://www.researchgate.net/post/Remove_an_author_from_my_article

So you will get access to the author list.

The order of authors can be changed by 'grasping' the appropriate author name by the mouse and shift the name to the appropriate place.

You may have to work on your issue in two sessions:

1) introducing additional names and saving the editing, and

2) editing again ( possible after about 24h) and then placing the names at the right order

good luck

Adding, editing, and removing co-author information is well described topic at RG Help Center page:

https://explore.researchgate.net/display/support/Authorship