1. Invitation via email address is not possible.

2. Give the room a suitable name (1). Make sure that encryption is enabled (2). Select who can access the room (3) and click on Create (4) in the top right corner.

Private rooms: Private rooms are closed and can only be made accessible to users by inviting them, i.e. by manually adding them. 

Public rooms: In order for a room to be publicly available via a link or a room address, a room address must be added (5).
This is composed as follows:
<#room-name>:<server-name>

When selecting the address at this point, you only need to enter <#room-name>.