When writing an e-mail in Roundcube, you can add multiple recipients for your email. To do that, start typing your contact info (search by name, surname, etc) in "Recipient" field, auto-complete suggestions will appear. Click the one you meant, then start typing next contact info, another suggestion box will appear, click the one contact you want.
You can add as many e-mail recipients as you like.
Kind regards,
Arek