On any user's page, to the right of their avatar, you can find the "Write a message" button. If it is not there, it means that the user preferred to limit the circle of people who can write to him.

However, it is not necessary to visit the user's page every time. In the "Messages" section (dancespace.net/messages ) you can quickly find a user with whom you already have a dialog through the search bar.

You can also create a conversation with several people there. Click on "New chat" above the list of dialogs and select the participants of the new conversation.

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.