400 people. Office 365. No one uses calendar invitations because default permission for everyone is 'reviewer'. How do I open open one of these calendars to look up something? I will neither sync with nor getting future updates from one of the colleagues calendars. And sometimes, but not often, I will setup a meeting with one or more of the colleagues.
Calendar invitations are not helpful. Or does it make sense for 400 colleagues to issue 399 calendar invitations each time? I don't think so.
We don't offer an easy way to "open" another user's calendar in the Outlook mobile apps. (Nor does iOS native calendar app either)
However, are you wanting to view other's calendars to see when they're free for a meeting? The Outlook mobile apps have scheduling assistant built in, so you can find a time that works for all the attendees. Just add people as attendees, and then tap on the date or time fields to open the mobile scheduling assistant.