Adding contacts to 365 groups

Hello, we have several contacts in our hybrid environment. These contacts serve as connectors to mail integrated cloud apps, like ConnectWise. We're in the process of migrating all of our distribution groups  to Office 365 groups, but contacts don't seem to be available as members. I read that contacts created in 365 with powershell can be added, but no they're not available either. Further reading indicates that 'guests' are the contact substitute, but guests are required to reply to invitations, so how does that work in this case?

Any thoughts are very appreciated,

Slosuenos64