I've been using JIRA for over a year now and feel comfortable with it. Now your Confulence seems to be a child from another father (or mother).
Why is that when I click on Invite users, I can find them but can't invite them? I see my Jira users but there is no action I can do. And I am Jira / Confluence admin!
Why can't I just allow all Jira users to be Confluence users?