A watchlist you keep on your own is a record. A watchlist your group can see is a decision waiting to happen. If you already track films on Letterboxd or IMDb, you do not have to type them in again.
Letterboxd and IMDb both let you export your data as a CSV file. Upload that file and the films are matched and added to your list. Ratings and notes can come along with them, and you decide whether imported values replace what is already there.
A Letterboxd RSS feed, a public list from the movie database, or any RSS or Atom feed of films can be added as a source that updates on its own. New entries appear on your list without another export.
A Letterboxd or IMDb export carries scores as well as titles. The import asks which member they belong to, turns them into that person's ratings and marks those films as seen. A CSV never states its own scale, so you confirm whether a 4 means four out of five or four out of ten, and the preview shows a real row from your file converted before anything is written.
Review text can travel the same way, kept as that member's private review beside the score it arrived with. Watch dates in the file stay with the ratings. Anything you have already rated here is left alone unless you turn overwriting on.
Films are matched by identifier where the source provides one, and otherwise by title and year. Anything ambiguous is shown to you rather than guessed at, and the match you choose is reused next time that source updates.
Once the films are on a shared list, your group can vote on them, see what is streamable, and put one of them on an actual evening.
Letterboxd offers a data export in your account settings, which includes your watchlist as a CSV file. That file is what you upload here.
Yes. IMDb list exports are CSV files and are read the same way.
It can. You choose whether ratings and notes are imported, and whether they replace values that already exist on your list.
A CSV import happens once. A feed source keeps updating, so use a feed if you want the list to follow the original over time.