[csunbird]

To be fair, when you are meeting under a place that is protected by magic (Fidelius Charm?) to make sure that only people who know about the secret place can attend the meeting (and this information can only be shared by a single person, the secret keeper), it automatically validates that the people joining the meetings are authentic.

[wizzwizz4]

Except that time it didn't, which every character in those secret meetings should be aware of. (There's no point trying to defend the plot holes in Harry Potter: it's an entertaining story and important to many people, but well-written it is not.)

[Izkata]

This was explained: The secret-keeper died, so everyone who knew the secret became the new secret-keepers. They brought the outsider with them accidentally, which was enough for the magic to be satisfied that the secret-keeper showed them the secret.

[wizzwizz4]

The other time it didn't (revealed in the third book) should be enough for them to at least have their guards up.

Though, given the events of the fourth book, every participant in the war must be incompetent at warcraft – which we do see in subsequent books. You could argue it's consistent with the characters being incredibly complex (traumatised veterans, ego-fuelled tunnel-vision, not actually caring about victory, etc)… but realistically they were written by a millionaire[1] with no experience of war, and are fairly one-dimensional characters.

[1]: Sunday Mirror, Jul 9, 2000.