In Chat, you have the option to create a chat message for one-to-one a group of up to 10 participants (including yourself).
To create a one-to-one chat, click Chat and then + button in the left side bar on web and + on mobile. Select the recipient and click Done. Selecting a single recipient will create a one-to-one chat message. If you select more than one recipient, a chat group message will be created. In the chat, you can star a message (long press on mobile), check if it was delivered and/ or read by the recipient (swipe right to left on iOS and long press on Android), and attach a file.
You can attach a file through Dropbox or any file on your computer. The maximum allowed size is 10 MB per file.