"share via" doesn't offer me mail as an option in some apps, though it does in others I want to use the (e)mail rather than the gmail app to share things, but in some apps this is not offered as an option. It is in gallery, but not in business calendar. It is in gtasks, but not in signnow. I use my phone mainly for business, so want to send invites to meetings and send signed documents from my work email rather than my private gmail. Is there a way I can add this back in? Have I got a preference wrong? Any help kindly appreciated. Thanks
This comes back to the individual app developer, you'll have to contact the developers of business calendar and signnow to update their share menu to be compatible with the Email app. 

On the mean time though, you can use Gmail to send as your work email. Once you do that, then you can share to Gmail, and pick the alternate From address by clicking on the From field on the Gmail app.