As long as you have your permissions set to allow your guildies to enter, they can go there when you are offline.

You can't change the size of the yard. What you see is what you get. Some people are building platforms etc to add more space, but you can't expand the size of the house or yard.

Also, the max number of players in a manor is 24, so you can't host an entire guild all at once.

There are building items in the game...planks, stone blocks, etc that you can use. Here is a link to another forum post that shows some of what people have been doing.

https://forums.elderscrollsonline.com/en/discussion/319458/my-first-home-show-me-yours#latest

https://forums.elderscrollsonline.com/en/discussion/319771/hunding-palatial-hall-rooftop-access-solution-pics#latest


The building items are available for crowns in the editor, but you can also buy them for gold from furnishing merchants in many of the larger cities.