You'll need 2 plugins. WorldEdit and WorldGuard. See our How to install a Bukkit plugin page on how to install them.

To protect a region.

1. Take out your WorldEdit wand. Type //wand if you don't already have one.
2. Leftclick one block.
3. Rightclick another block elsewhere
4. 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.

1. 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)

1. 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.