As Natalie pointed out in her excellent tutorial, New Residents Guide To Buying Land, with a premium membership you may own a 512 sqm parcel without incurring an additional monthly "land use fee". I have a (very sweet) couple living next door to me who each got a premium membership and who each bought an adjacent 512 sqm plot. Pretty smart! But to make it more convenient to build for 1028 sqm, they first "joined" the two lots (more on this later), and turned it into "Group" land. Sound's simple, right? But alas, group owned land is a little tricky.
The first step is to create a new group allowing only your most trusted partner(s) to join, or at least hold a powerful position, such as Owner or Officer. This is because once the land transfers to the group, anyone holding such office may dispose of it, or even sell it to themselves. Suffice it say, be careful with this!
Once your group is established, you may "donate" the land to the group through the About Land, General tab, Group setting. Set the group to your new group, and check "Allow Deed to Group", then "Owner Makes Contribution With Deed", then hit the Deed button. When this is done, you are no longer technically the owner of the land, the group is. Oh, but you still get to pay the monthly "land use" fee if your total holdings, including the donated land, are over 512 sqm. You agree to this when selecting "Owner Makes Contribution With Deed".
You may now build on the land as normal, but with one caveat. In order for other members of the group to move, change, or delete an object, that object must be set to "Share with group" through its Edit, General tab. Forgetting to do this can be the source of much frustration. Also, there is no reason that I know of to actually deed an object to the group unless it interacts with the land, such as a radio or other streaming media device. It's a good idea to have your group "activated" in your profile when building there.
If you do end up deeding an object to the group, make sure you first have that object's "Next owner can" permissions set appropriately. This includes items in the object's contents, such as notecards, etc. Failing to do this can result in permenant loss of copy/mod/transfer rights to the object or its contents. To retrieve a deeded object, simply sell it back to yourself; the same "Next owner can" permissions apply.
Also, never leave the group without first retrieving your land and object donations. I think the easiest way to do this is by simply selling the land or object back to yourself. You can safely set the land for sale to you specifically for $0, but not so for an object (so act quickly lest some hovering bot grab your object cheap).
Joining Land
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You really don't have to join the group's land into a single parcel unless you want to ensure uniformity of land attributes, or to accommodate such things as coverage of the entire land by steaming media (like a radio) or a single security device. Note that the separate parcels must be in the same region to proceed.
Turn on "Property lines" in the View menu, then right click on the ground where you can see the land boundary separating the two parcels. Select Edit Terrain and drag the yellowish select box anywhere across the two parcels (over the boundary). Now just click the Join button in Edit and you're done.
Note that you can always un-join land, too, as the need arises, but I won't go into that here.
