Disabled on singleplayer by default.
If you die, you respawn on your island, if you leave the team or it's deleted, you get teleported to spawn. If the team is deleted, the island isn't. Islands are placed at center of each region file, to minimize worldgen and make it easier to clean up later. For each new team a new island is placed in spiral pattern. If you delete a team and re-create new one with the same ID, a new island will still be placed.
Island templates are loaded form the same location as vanilla templates for Structure Blocks (world/structures/teamislands_island.nbt. If you want to create your own island template, you can use Structure Block in Save mode, select area and set ID to teamislands:island and press SAVE button on right side.
Lobby isn't loaded from a template and simply is the spawnpoint of world, though I might later add loading it from template later.
By default players spawn on top of template in center, but relative spawn position can be changed if you add "spawn_x", "spawn_y" and/or "spawn_z" NBT tags in the template .nbt file (You can use NBTExplorer to edit it).