How to create a room list in Office365

To let the rooms show in the room finder, you need a room list which contain these room mailbox. You may check the steps below for setup.

1. Using administrator account to connect to Exchange Online through PowerShell. 
2. Create a new room list which share same attribute like building or floor,
   
   New-DistributionGroup -Name "Building 1 Conference Rooms” -RoomList
   
   
3. Add room mailboxes of building 1 into the distribution group,
   
   Add-DistributionGroupMember -Identity "Building 1 Conference Rooms" -Member roommailboxname

Then when you create a new meeting, select the meeting time, choose a room list in the room finder and you will see the room mailbox listed in the available room.

Due to the Offline Address Book update interval, you may need to wait up to 24 hours to get the room list.

See also:

https://technet.microsoft.com/en-us/library/jj215781(v=exchg.160).aspx