Accounts granted in AWS (with IAM) are independent of user accounts on a given instance. IAM account give the user access to parts of AWS defined by the policy associated with the group or user.
If you want to give someone access to the instance, you can have them generate their own personal keypair (ssh-keygen or puttygen). You will take the public part of the keypair and add it to the ~/.ssh/authorized_keys
file of the user you want them to login as.