Set-DistributionGroup -identity GROUPNAME -managedby @{Add=SAMACCOUNTNAME}

The group has one manager already and one co-manager already.

When I ran this command

Set-DistributionGroup -identity GROUPNAME -managedby @{Add=SAMACCOUNTNAME}

Posh returned an error message

"

All groups must have at least one owner who manages membership, message approval, and other
settings for the group. Make sure you assign an owner for this group.
    + CategoryInfo          : NotSpecified: (:) [], RecipientTaskException
    + FullyQualifiedErrorId : A2FA9887
    + PSComputerName        : srvrname.domain.com

"

I just needed it to add another managedby co-manager like TechNet states at

https://technet.microsoft.com/en-us/library/bb124955(v=exchg.160).aspx