one goal of the verified groups are the protection against active network attacs.
to reach this goal, each cryptographic setup of each member has to be verified with a second factor by at least one member, currently, this hast to be a qr-code-scan. maybe there will be other options in the future.
however, as said, this verification is not needed by every member - so in your case, maybe someone else can add the new member?
if this is not possible, you cannot add the member to the group as it is not guaranteed that the cryptographic setup is not compromised.
nb: as a workaround, i personally also managed to do a qr-code-scan during a video-conference, not sure, however, if that mitigates the seconds factor and may introduce new attac vectors.
if the members are already verified, this is on-purpose.
in general, please note, that, although already well-usable, verified groups are still in “experimental” state.