When I send out invitations to a private blog can they be asked to fill something in so we can distinguish comments from ‘small’, ‘diaspora’, ‘African counterparts’ and ‘other key stakeholders’. Is there any way we can do this?
There is no functionality for this within Blogger's(*) invitation-to-participate process: you cannot customize this email at all.
But I guess that as an adiministrator you could set up a Google Form, and when someone accepts the invitation, send them a follow-up email welcoming them and asking them to complete the form, and then manually using the results to update a profile you have for them.
Yes, this is more work. But since you can only ever invite up to 100 people to each private blog, it may be manageable.
(*) since you used the Blogger tag, I've assumed that you mean Google's Blogger product. I don't know what options other blogging products may have.