You can manually add users to the site without being a manager and without inviting them. Log into Share as admin and click the More menu at the top of the screen and go to Groups. From there, check the box that says "Show System Groups" and click the Browse button. You should see groups listed for every site in your repository. Find the group for the site that you say has disappeared, and then you'll see the subgroups appear for the site. There will be a subgroup for each site role (consumer, contributor, collaborator, manager). You can add a user to the manager group for your site, and the will automatically be a site manager without having to invite them.
Hope this helps.
