These users should also be able to invite outsiders, users that are not a member of the domain
The planner will currently not do what you need as it can only send emails to openfire users or group members that have email addresses. It also requires you to configure your openfire email sender and can use the provided calendar for a one-off meeting or a CRON trigger for a re-occurring meeting like every Tuesday at 4pm for example. It will do your internal users, but not the external ones.
I then want to install jigasi and connect our PBX.
That is work-in-progress and not yet available.
I also need to enable the Screensharing for firefox
I don’t use Firefox and cannot confirm that for you.