After you have secured a place in a match, you'll gain access to that match's chat. The match chat lets you send messages to other players to arrange details such as time, location, or any special requirements.
All messages in the match chat are visible to both current and future match members. This means that if a new player joins later, they can still read the conversation history from before they joined.
Outside of match chats, Grands does not currently support direct one-on-one messaging between users.