My users are only allowed to have 1:1 conversations. They are trying to add more persons to their conversations even if they are not allowed to do so. First the phrase "Separate names with a comma. You may invite up to 1 members." let them believe they can add another to the known 1:1 conversation. Second the option "Allow anyone in the conversation to invite others" let them believe they can actually have more participants in the conversation. For 1:1 conversation settings it would be best to simply remove both areas. Fix for removing the second option is to add this in extra.css:
I have adjusted the initial text box if they can only invite one person.
However, the second part relating to the option for inviting can't be changed generally, as it reflects on the participants and it is not known whether they're able to invite. If that's applicable to you, then I recommend editing the template or using that CSS.