You must do this with one other player in an online game.
1. First, make sure you are on the other player's friend's list, and they are on yours.
2. Have both players load up Zoo Tycoon and wait at the main menu.
3. From the main menu, have one player select
Start Online ->
New Game ->
Start Online ->
Sandbox Zoo ->
Select ANY Zoo ->
Confirm Selection.
4. The game should prompt you to invite friends to your zoo. Choose
Invite Friends Now. Then select
Add Friend and select your friend's gamertag. Then select their gamertag again and choose
Invite to Zoo. This should unlock the following achievement for the player who created the zoo and is doing the inviting:
5. If you are on Xbox, you should get a game invitation notification. Simply accept the notification and you should join the other player's zoo and unlock:
If you are on PC, you will have to select
Start Online ->
Join Zoo ->
Select Your Friend's Zoo and join from there.
6. Simply build 15 items. There is unlimited money in sandbox mode, so the only thing you'll need to wait on is the loading and transition times. The attractions can be the same. I simply built 15 of
Alpine Tree Small A, since it is already unlocked from the start of the game, and you don't have to connect it to anything or go back through the menus again. Once you build any 15 attractions, both players will unlock: