You can just start the scene and invite them. The system will send them a notification that they've been invited. You can make it public or private, either way. And if there's something you want to say through OOC, you can put it in the OOC messages box for the cutscene.
If you're worried about a thought like "but maybe so-and-so does not really want to cutscene with me"... try not to worry (I know that's hard with anxiety, but most people do want to roleplay, that's why they're playing this game)! If they don't want to be in this scene, or would not be in this scene for some reason, they can just not post. If the scene is only with them, they can click End Scene and it'll be gone within the next five hours. When the cutscene log shows up in their logs, they can just unsave it, and it'll get purged along with all other logs that were unsaved or never saved.
It's fine to end a cutscene or not post without much need to communicate it -- just like in-game interactions, sometimes a person does not have the time, or sometimes a person cannot be found easily.
There is one particular cutscene etiquette that I'd like to mention though...
It's fine for a cutscene to move as slow or as fast as necessary for those involved, HOWEVER: Please take into account the ongoing plot of the game, and try not to awkwardly overlap time. Here's an example: Jerembeli has a friend named Josaf and a friend named Jalladin. Josaf usually plays evenings, Jerembeli plays mornings even though they are evenings for him due to timezone differences. Jalladin plays pretty much all the time. Jerembeli wants to tell Josaf and Jalladin, together, something important before the RPT in three days (Sunday). It's Thursday, and on Saturday morning, Jerembeli and Josaf will both be playing in-game.
Let's say the cutscene starts and is pretty slow because the three OOC friends are carefully taking turns and writing long interesting play-by-post style blurbs. We get to Saturday morning. Jerembeli and Josaf are both online. Maybe they want to play with each other. But they feel awkward about it, and awkward about playing with Jalladin too, because ... they're in the middle of a scene that theoretically started three days ago, but is unfinished. Who knows what happened at the end of that scene?
Or let's say that the RPT comes up. They still haven't finished the cutscene, and Jerembeli hasn't been able to tell his friends the important thing, even though he really had multiple opportunities.
Because of this discrepency between cutscene-time and in-game-time, we need to be cognizant of time frames when using cutscenes.
It's fine to quickly close out a cutscene with a summary post like: "Hush!" exclaims Jerembeli in frustration. "Look, I gathered you both to tell you something important! The doomspawner will be descending from the sky at noon! I had a dream about it! You can believe me, or not!" Jerembeli turns and runs away. And then end scene. It's fine to post "out of turn" in a cutscene. It's fine to have short cutscenes too.
Some cutscenes might be ongoing things, like ... a doctor, checking on a patient every day, and every few posts it's a new day. In that case, if the patient sees the doctor between one of those posts, they've really just seen the doctor between checkups, and the previous checkups can exist in the memory of the patient. And maybe the cutscene only ends when that small plotline ends, or when there's enough of a shift in the routine that it deserves a new log.
But for cutscenes that are a single scene, like "Jerembeli sits and discusses the doomspawner with his friends", it would be awkward during in-game time for Jalladin to be telling his other friend, Jamarla, "Jerembeli said he had an interesting dream." and Jamarla goes, "Oh yeah?" and Jalladin is like "Yeah but he didn't tell me what it was..." And then in the next cutscene post Jerembeli explains the contents of his dream, and at the end of the chat with Jamarla, Jalladin says, "Oh yeah, Jerembeli told me his dream now, the doomspawner will be descending from the sky at noon." Like... when? When did Jerembeli say that? This sort of thing creates a parallel timeline issue that we want to avoid. So for these single-scene cutscenes, either don't roleplay about the fallout until the scene has finished, or don't consider the scene to have taken place at all until it is finished.
Because of this rational stipulation, it's nice to be aware of fellow-player's time-based needs in cutscenes and try to finish them off in a tidy fashion. The cutscene system is very flexible and this is probably the only etiquette I'd consider to be important -- to try to take the timeline issue into account, and be careful with it. During my attempts to play async MUSHes, my main difficulties in following the story have been this timeline issue, so that's why I'm emphasizing it here. Song is a story focused game and we want the story to make as much sense as possible.
Edit: Took me a long time to write this post (through some RL distractions) so I didn't see Zahra's post up there. Of course, it's not necessarily against etiquette to send an OOCMail if you are worried or if you just feel more comfortable doing so. But it's not required to do, and I don't want it to feel like a cultural requirement to ask people to roleplay.