Can a user belong to multiple groups?

A, Yes

http://www.aiotestking.com/amazon/can-a-user-belong-to-multiple-groups/

A group can contain many users, and a user can belong to multiple groups.
Groups can’t be nested; they can contain only users, not other groups.
There’s no default group that automatically includes all users in the AWS account. If you want to have a group like that, you need to create it and assign each new user to it.
There’s a limit to the number of groups you can have, and a limit to how many groups a user can be in. For more information, see Limitations on IAM Entities and Objects.
http://docs.aws.amazon.com/IAM/latest/UserGuide/reference_iam-limits.html