Member not opted in cannot be added to Agenda

We have a member who joined, but has not opted-in via email.
This member was a Grammarian, but I cannot add to the Agenda since member is not 'visible'.

I thought of deleting the member and adding without email address. When I tried that the name was already in use.

You need to un-delete them and get them to opt-in.

What you could maybe do is just reserve their spot on the agenda (maybe put yourself there, for example), resend them the opt-in just before the meeting so that it is near the top of their inbox, then take your laptop or device w/ you to the meeting so that they could log into their email w/ your device/laptop and click the link to opt-in. I realize that may seem a bit cumbersome, but these are the times when data privacy is driving a lot of this unfortunately.