To protect a region.
- Take out your WorldEdit wand. Type //wand if you don't already have one.
- Leftclick one block.
- Rightclick another block elsewhere
- Type /region define MyArea into chat.
Now only you and Ops can modify the area between those blocks you selected. Keep in mind blocks below or above the 2 blocks you selected won't be included in your region. Be sure to select one low block and on high block for the region.
To allow a player to modify a region.
- type /region addmember MyArea Bob
Now Bob can modify the region titled MyArea.
To allow a group to modify a region. (The group must be listed in Permission group)
- type /region addmember MyArea g:Mod
Now Mod's can modify the region titled MyArea.
For more info on protecting regions, see WorldGuard's How to protect areas page.