we have this problem in our book club and in tv series discussions all the time, and it really isn't a problem. everyone shares their thoughts to the things they are reading/watching and people give their feedback. some people avoid spoilers until they are caught up and add their thoughts then. some people are behind and talk about things we already discussed days ago. i even once discussed a tv series season one while everyone else was already watching season two. we didn't get intodeep discussions but people were answering my questions without spoiling things for me. certain movies are being discussed over and over again every few months or years. if you have a group that communicates well with each other, none of this matters. the worst thing that can happen is that latecomers get less engagement. but it really depends on the detail or the mood.

a system that allows marking of spoilers with the chapter/episode/part etc, and hide the ones that are further along than me would be nice. practically speaking though it would be enough if everyone could tag their messages with the chapter/part they are talking about and then the reader can choose to unhide based on that information.

the hard part with any such system is to get the participants to develop the discipline to actually recall the chapter and mark the comment. "somewhere in the book the gardener removed a sticky substance from his shears, but i forgot which chapter that was."