How do alliances work?

[Poot Pootington 30]

Here's the thing, I don't mind sharing access to our smithy, loom, shipyard, etc, but I'd rather not have ppl be able to build on our claimed land and kill resource spawns.  

If we form an alliance with the companies on the island, can we share access to shipyards and such to work together towards large projects? Can Allies access sections in a ship to help sail?

Or would be better off trying to form one larger company?  I know you can PIN code access to chests and such within a company, but can you restrict the ability to build in a claim?

Then there's the whole Bank, taxes, and lighthouse mechanics that confuse me even more. 

Basically, can I have my cake and eat it too?

[Goldfish The Builder 105]

Larger company would always be easier.

Ally would be better if your share an island but want to be friendly with those sharing it.

If you want to build and sail with them I'd suggest a merger.