I am wondering whether it is possible to limit the possibility to vote in a poll to certain groups. Should that not be the case, I figure it would be easy to add that functionality, would it not?
Sure, create a category, give only that group permission, create a poll in that category 
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I specifically asked about the right to vote instead of access, because some processes should be transparent but taking part in them should be limited. Sorry for not clarifying earlier.
Do reply permissions include those for votes? Then disabling those in the categories in question is not nice, but it could work.
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Per concludere questa questione, è possibile farlo da tempo. 
Puoi scegliere quali gruppi possono partecipare a ciascun sondaggio utilizzando l’opzione Limita il voto a questi gruppi nella sezione avanzata:
Puoi votare in questo sondaggio?
- Sì
- No…
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voters