How can I create a chat for several people?

There are two ways to create a chat. Open the "Messages" section (dancespace.net/messages). If no dialog is open, you will see the "New chat" button right in front of you. Enter the user names in the "Recipients" field.
Another way is to simply click "New chat" above the list of all dialogs.

If you don't see the person you want to add, start typing the name of the new participant in the recipients line.

There cannot be more than 500 participants in a conversation. And one more important detail: you can only invite people from your friends list to a chat.