In some topics it’s not possible to invite another user to the conversation. This happens when it’s in a category that is not open to everyone, which might make some sense in the case of something like a Regulars Lounge category. However, if a category is just not visible to anonymous users (but even a TL0 can Create / Reply / See in it), that already prevents inviting other users to the topic.
I think the logic being applied is to prevent someone from sharing a link / invite by email (which makes sense) but is also preventing inviting existing users by their username.
So instead of:
Does this happen for all users?
Users with a TL0 or TL1 cannot invite others and won’t be shown the invite link. Only TL2 or above can send invites and get shown the invite button.
I reproduced this as staff and it seems to be dependent on the category permissions.
Edit to add: it also happens to TL3 and we use the default invite settings.
I can reproduce this on my local development site. Even as an admin user, when I set a category’s security permissions to Trust Level 0 can Create / Reply / See, the share dialog doesn’t give me an option to send an invite. I think that Discourse is trying to prevent invites being sent to topics that a user may not have permissions to access, but all users on the site will have TL0 status, so preventing invites from being sent doesn’t make sense for this case.
Aha! possibly an easy bugfix to be scheduled next week @eviltrout?