I have some questions about joint property ownership when buying property jointly with others (not with a spouse, but other unrelated people):

1) How does one know if the shared ownership is an "undivided share"? Is there a de facto in law?
2) It there such a thing as a "divided share"?
3) If two parties own a property as specified 50% each on the title deed, do all decisions about the property have to be jointly agreed or can one person do something such as pave a garden, install a new door or paint a different color without the other parties agreement?
4) If a share is an "undivided share", can the shareholder legally sell his share without the agreement of the remaining shareholder?

My family entered into a 50% joint property ownership with another party many years ago. The did all the arrangements with the other owner on a handshake as such. I then bought my family's share from them about ten years ago with the agreement of the other party, and we continued operating things with the other owner on a handshake as such.

The other shareholder has now on-sold his share without my agreement (can he do this?), to a person who wants to make modifications to the premises; and also wants to draw up a shareholders agreement (shareholders agreement is a good thing).

What are the rights of 50% shareholders? Do they have to agree on any decision for said decision to be implemented? What if they cannot reach an agreement?