adding email adress/people into contact group (issue)

Hello everyone.

I often create contact group. I used to be able to put any email adress/people I wanted into these contact groups...until today.

I created new group today, and for some reason, I sometimes get a message saying "you cannot add this user to this group". This message seems to happen only for specific people that doesn't have a smartsheet account. I however used to be able to add people without smartsheet, so I don't really know what is going on.

Has anyone else noticed this new restriction, and does anyone knows how to get rid of this restriction ?

Thank you