We use Google Apps, and we have many groups of recipients.
Is it possible to send an email to a group, and exclude a certain recipient from that group? There is such an option in Outlook; is it possible in Gmail?
Saariko, the short answer is no.
I assume you're talking about domain-wide Google Groups, not a group of contacts for a specific user. Since Google Groups is a separate service (rather poorly integrated into Gmail), the full membership of that group cannot be expanded and modified in the To field like it can in Outlook with Exchange.
However, an individual user's contact group will expand into individual email addresses when typed into the To field, and you can then remove any you wish. If the population of your group was relatively inflexible, you could mimic the domain group with an individual contact group to provide this functionality.
If you click on the Gmail button and choose Contacts then choose your group, you can choose all members of that group and then selectively uncheck the ones you don't want.