Invitations in Free Version

We use both inside and outside people on the team. We are using the free version. The 15 user limit is fine for us. A few questions. People are not seeing invitations. Can I see pending invitations? We are using a workspace. Would an organization work better? Can I convert from a workspace to an organization?
Thank you!

I think I asked this backward. Can I change from an organization to a workspace and have everyone use gmail or outlook etc. email addresses.