Answer: After creating the database availability Group (DAG) the next step is to add mailbox servers to the DAG group which will allow us to have multiple copies of our mailbox databases spread across mailbox servers.
If you need to create a DAG please use this manual.
In order to add members to our DAG, we need to follow the following steps:
Open Exchange management and go to Organization configuration -> Mailbox and click on the database availability Group.
Once there right click on the DAG and click on manage Database availability Group membership.
Click on add and pick your Exchange 2010 mailbox servers to the group.
Once completed all you have to create a copy for each of your mailbox database.