From the conversations screen, choose the conversation you want to mention someone in. Click the square icon at the bottom left of the screen to enter expanded comment mode. In expanded comment mode, you will have a larger window to view your typed response and options for @mentioning, marking the comment urgent, and/or adding a picture to the conversation.
Click the @ symbol to access a list of people and groups you could mention. Use the search field to find a person or group specifically. Press the person or group you want to mention and you will be brought back to the expanded comment screen with the person or group highlighted in light red. The light red highlight indicates that they will be @mentioned in the comment.