five issues with one Office 365 Group

Have any of you seen this issue before?

I am building a private group in Office 365 Outlook. I can add Office 365 users just fine, and I've been able to add many guest members (using email addresses outside the domain). 

But, for a couple of the guest email addresses, when I hit "Save", I get the error "The group can't be saved now. Please try again later." 

This only happens with a couple specific email addresses, I know this as I was able to add 20 other "guest" users with email addresses outside of the group domain. I've also tried to add these same email addresses to other groups, and I get the same error.

for the two email addresses I am trying to add, they are from different domains and formatted differently. One is mailto:fname.lname@xxxx-xx.com the other is mailto:flllll@xxxx.com.

Any help would be appreciated...r/Chuck