It was interesting to see when I was trying to add a member to a “mail enabled” security group in Exchange Admin Center, I get this:
Oh! I was freaking add a member…what the?
Well, we have two options.
One: Add the member using Active Directory Users and Computer console. It is easy, but we are not fixing the under laying issue. So the option two is necessary.
Two: Make the group closed so members can’t leave themselves (even freaking though nobody is trying to leave the group) so we don’t get above message. Open PowerShell and connect to your Exchange Server and use the following cmdlet to close the group.
Set-DistributionGroup <Group Name or Alias> -MemberDepartRestriction Closed
Did it help? Leave me a reply.
Anand, the Architect 
Thanks Anand this solved our problem : )
In order to set the member join or member depart restrictions to ‘closed’, you must set both properties simultaneously. I.E. Set-DistributionGroup %DGName% -Memberjoinrestriction Closed -Memberdepartrestriction Closed.