When you create the channel, you can add users by entering their names. You can also do this afterwards by clicking on the user icon in the top right and then selecting "Add user". However, this button is only visible to users with the "owner" role (that is, the person who created the room and other users appointed by them as owners). As an owner, you can also generate an invitation link via the members menu and share it with other people who are registered on wechange.de.
As the owner, you can also type “/invite @username” into the message field and send it to add a person (of course, you have to replace username with the profile name).