Unlike 1 on 1 messages, rooms don't give an audio alert whenever there is new activity. Using the desktop client for Windows or Mac will help you keep track of new room messages, but you can make sure a user (or users) see a message by @mentioning them.
To @mention someone, type a @ symbol or click the @ button next to the text box. A list of room members will pop up*. Select the name of the person you want to @mention, then type and send your message as usual. To @mention more than one person, continue to tap the @ button and add as many people as you like.
You can use All Members to notify all room members, whether they're online or not, or All Present Members to notify the people who are present in the room at that time.
The @mentioned person (or people) will then receive an audio and banner alert just like a 1 on 1 message, and the name of the room will be mentioned in the banner.
*On Android HTC, you may need to start typing the person's name.