Active Directory – Mailbox Rights

I have an employee who needs access to a User Account in Active Directory that is used to manage departmental emails. This employee happens to be a part of a Security Group. The Security Group has over 90 members and I want to give the whole groups Mailbox Rights to the User account, but when I try it, it doesn’t give the employee the rights. What I ended up having to do was add the employee’s User account to the User account I want her to have Mailbox Rights to. Is there any reason why I can’t give the Security Group Mailbox Rights to the User Account?