Why am I unable to add members on my project page?

I have started a new project but I'm unable to add any new members in it. When I type the name in the invitation box, though the name appears but the member is not added. Is it just happening to me only or all the members?
2 Answers

For information only
- The project seems to be "private"
- You can add users from the preview (not necessarily in the editing model)