This article is aims to describe the detailed steps to add Mailbox server to Database Availability Group (DAG):

1. Adding Database Availability Group Server:

Add-DatabaseAvailabilityGroupServer -Identity <DAGName> -MailboxServer <ServerName>

2. Adding a copy of Mailbox Database to the new DAG member:

If you want to add a database copy from any active database on any mailbox server:

Add-MailboxDatabaseCopy -Identity <DatabaseName> -MailboxServer <DestinationMAilboxServer>

If you want to add a database copy from a specific mailbox server:

Add-MailboxDatabaseCopy -Identity <DatabaseName> -MailboxServer <DestinationMAilboxServer> -SeedingPostponed
Suspend-MailboxDatabaseCopy -Identity <DatabaseName\DestinationMAilboxServer>
Update-MailboxDatabaseCopy -Identity <DatabaseName\DestinationMAilboxServer> -SourceServer <SourceMailboxServer>

3. Check database copy status:

Get-MailboxDatabaseCopyStatus -Server <DestinationMAilboxServer>

i.e. coping mailbox database process takes time depending on the database size.
i.e. don’t forget to modify activation preference number of database copies.