What Army or DoD policy addresses unprofessionalism in a group chat? Can someone be counseled for disrespecting an officer in a group chat?

Group chat (if military groups for purpose of sharing work requirements) conversation carry with then the same as if in a room physically, there is no fire wall of safety. If the officer believed you knew that he was an officer and knew your grade he can of course counsel you and recommend whatever he likes. If the Company Commander agreed he would forward the request for the AR15 packet. From there he would have to speak to you, he may before, in order to determine the facts before rendering a secision, which you can appeal if you feel you have been wrongly judged.

Is he petty, not petty, feeling disrespected is what will be evaluated.

There is something to the fact that many folks feel more comfortable saying things in a chat as apposed to in person. This should be a lesson learned regardless of the outcome. Maintain your military bearing at all times.

To my knowledge, there isn't a specific U.S. Army regulation that explicitly covers conversations in group chats on messaging platforms. As CSM Darieus ZaGara stated, you would conduct yourself online as if speaking face-to-face with the person or the group. In this regard, I think AR 600-20 (Army Command Policy) is a relevant reg, and the broader principles of maintaining professionalism would logically extend to how soldiers engage on social media platforms, including group chats. Army Command Policy doesn't provide detailed guidelines specifically focused on online interactions or group chats but does address the principles of professionalism and maintaining good order and discipline within the Army, which can extend to online conduct.

SPC (Join to see), most Senior Leaders have not had to deal with Social Media on the way up to where we are now. But dealing with technology is continually evolving. First, nothing can replace face to face conversations. However, the foundation that our tenets of professionalism are built on do not rely on understanding who you are talking to. Ask a Signal NCO who has had to manage a signet.

The unfortunate thing is young leaders have taken to use text messaging and apps like signal to replace or significantly supplement normal verbal orders and conversations. The unfortunate thing for young Soldiers is that they treat them like SM or when speaking to a colleague or friend. No matter the platform, "I am a professional". While "LMAOOOOO" in a text message may not seem like a big deal to you, the LT and probably the SGT must have felt a certain way about your lack of "taking it serious", especially given the context.

Most of us have been counseled for any number of minor infractions from talking back, smart mouthing, asking too many questions, sock drawer in disarray. They, individually, reflect the culture that leader wants to install. Is everyone 100% in line with Army values or leadership models? No, but they have the authority to enforce standards and the flexibility to be creative, to an extent.

The thing about counselings, especially the ones we might classify as "trivial", is they really don't matter when you PCS or get new leadership. I have taken previous leader counselings with a grain of salt, especially if they are written poorly.

Now, if your example was included because the LT was requesting administrative punishment, we might have a conversation about why you think professional conduct standards would not apply on Signal or if you think replicating the typed words in person is appropriate. I think laughing out loud after instructions given by a duly appointed NCO or Officer would garner a reprimand of some kind in any setting. If you did not walk away learning what the leader thinks about that, they did a poor job counseling you and the form was a way to slap your wrist. An improper use of the counseling process, IMO. My recommendation to the Commander would be ODP regarding mentorship and counseling and let me and your 1SG handle you. I could see how unprofessionalism could be constructed as disrespect to an officer, and that would be the second focus of our conversation.

All that aside, checking disagree does not do anything. By checking that box, you are disagreeing with anything that is incorrect on the counseling statement including the character of the conversation, plan of action, leader responsibilities, key points of discussion, or any errors that may be contained in the administrative portion. This is not your opportunity to grandstand about how you think the leader is wrong in counseling you in the first place. If your feedback to the counselor was not captured in the key points of discussion, you check agree and add in your remarks your side that was communicated during the counseling but left off from key points.