User story

Is your feature request related to a problem? Please describe.
Acording to #5861 (Conversations with more than one person) issue, there's some improvements to be done.

Describe the solution you'd like

• There are some aria roles missing from my previous example that could be useful in this case, like aria-owns (that links the input to the autocompleting options), aria-expanded (that tells the user when a control is expanded or not) or role="option" to indicate that somehing is an option in the expanded menu.
• It looks like you can select the same user several times, shouldn't the selected ones be removed from the autocomplete list?

Additional context

🎨 Frontend

Does this issue could impact on users private data?
No