This may be a restriction of current Roundcube architecture, but in case it is not, let me file it...
- Ticket Type
If you're in the File section of the web client, invitations about being invited to a collaborative session show up.
If you're in the Mail section, for instance, then there is no invitation pop-up.
At least that was how @seigo and myself found it on demo.kolab.io yesterday when recording the screencast.
This works for me. So, it may be some timing issue. Note that these notifications are not "instant", they are "pulled" in one minute intervals (timer starts when the page is loaded).
ps. it's Files section, not File section. That wasn't precise ;)