Do you want to add guest users to Office365 group?
Please note that the UPNis a type of stringinstead of an Email address.
It is impossible to add an external user to O365 Group, but you should get its UPN.
And there is no action could get UPN of an external user, so the only way you could do is store these user UPN into an Excel table. When a response submits, use Filter array action to find corresponding UPN, and add it into the Group.