I also don’t feel it’s of any benefit for my team.
You think there is no benefit in a coach? What are you, perfect? "Inspect and adapt" is the core of Scrum and you are flat out rejecting it. It would be good if you could embrace it instead. Let them do their jobs, think about their suggestions and improve your team.
Because let's be clear about one thing: there was no stray agile coach that followed you into the building from the bus stop one day when it was cold outside and you left the door open for too long.
Your bosses have decided to hire a coach.
You will be coached.
Or you will be out of a job. That's your two options.
From a purely Scrum perspective, there is two core values that come to mind here: Transparency and Respect. For most ceremonies, there is nothing to hide. It's work you do for the company, not your private party. The planning, the dailies, the review, all of them are just meetings for work, not secrets to hide.
One exception may be the retrospective, as members have a right to know who gets to know about the contents of that meeting. I have often seen something called the "Vegas Principle" used for retros: what happens in Vegas, stays in Vegas. Meaning: don't talk to people outside the retrospective about what happened in the retrospective. But if the coach attends the retrospective and adheres to the same principle, then it's still open and respectful.
Now, sometimes a coach can be disruptive. But you have no choice whether there is a coach or not. You can influence the rules by which they play. For example, the coach may not want to announce beforehand which ceremonies they attend, to actually get a valid picture of the current situation, instead of a staged ceremony where everybody knows "this is for the coach". But I think it would be absolutely okay to ask for some respect to not disrupt the meeting and to have even the coach be there on time and don't leave early. So if they attend, they attend like everyone else. No coming late, no leaving early.
Your coach should come to a team meeting before they attend any ceremonies to introduce themselves, explain why they are there and what they are going to do, so you don't have to do that the first time they attend a regular ceremony.
Under what circumstances should outsiders attend a scrum team’s ceremonies?
As needed. And the need for a coach has been established by your boss.
We often had people from sister teams attend dailies or plannings to communicate their state or offer help with their product we had to use. Sometimes we were requested for their dailies or planning to make sure they got it right and we could support them adequately. Sometimes a manager would attend, silently, just to see how the people they manage actually work.
I have attended other teams dailies just because I worked with someone on a problem in their teams office and the time for the daily came and it would have been super strange to go into the hallway, close the door and stand there until their daily is over. We may be different Scrum teams, but we should all be in the big company team. A daily is just work, no secret sauce you need to protect.
If your team is afraid of outsiders silently attending, then either your team needs a stronger self-esteem, or your company is doing something wrong that your team is afraid of performing their jobs publicly.
Either way that's a problem the coach could address. With your team or with your bosses. They paid a lot of money to them, so the incentive to listen to the coach is higher than the motivation to listen to you. Sounds silly, but that's the way managers think. Use that.
Embrace the coaching and use it to improve yourself, your team and maybe get some improvement done on higher levels that your own feedback cannot reach effectively. This is a chance, not a nuisance you have to dodge somehow.
