Both Approvers and Guests can use messaging. Your role only affects what you can do inside albums and photos, not in messages.
Using messages
1
Open messages
Click Messages in the sidebar to see your conversation threads.
2
Start or continue a conversation
Each project you’re invited to has its own thread with the owner. Click on a thread to open it.
3
Send a message
Type your message and press Enter to send. Both you and the owner receive in-app notifications for new messages.
Messages are 1:1 between you and the project owner. Other project members cannot see your messages.