Google Group Membership Add problem

[PACCofGCAdmin]

For my Google Group, I am unable to add a member, that is not in the membership list, but when I add, this error indicates the email is in the membership list, although it is not findable in the search function, "An error occurred 1 email address is the primary, secondary, or alternate address of an existing member (email)"

Please advise and Thanks so much!