NOTE: Attachments are not available through "Messages." You can send attachments via Chat, however.
From the Home tab, select the "Team" you wish to manage.
Tap Messages.
Tap the + Icon.
Tap To.
You may select All Players and Staff or select Individual Teammates.
Once all desired recipients have been selected, tap the Checkmark.
Tap Subject and input all desired information.
Tap Message and input message body.
Once satisfied, in the upper right-hand corner, tap the Send icon