The illegality and buffoonery of the Signal group chat, in order of severity:

1. High-level administration officials (only some of whom are reckless drunks or amateurs who accidentally stumbled on-stage) held a “Top Secret / Sensitive Compartmented Information” (TS / SCI) meeting outside of secure government channels. This, on its own, is a violation of the Espionage Act.
2. They set the messages to disappear after a week, meaning that they were retaining no history of their actions. This is illegal under the laws that require the preservation of official records.
3. They accidentally invited a reporter into the treasonous and illegal chat.
4. They have offered shifting and easily falsifiable excuses for the entire incident: it was a hoax, they were hacked, no secrets were discussed, and it wasn’t really that big a deal. The whole government is now running on nuke-secrets-in-the-bathroom levels of security.
5. They used emojis.

That last one should probably be higher.