It is possible that the recipient of an invitation will report that they cannot accept the invite, since the button is greyed out.
If you encounter this behavior, make sure that there is no other drive setup using the same name as the drive you are currently trying to accept the invitation on. You should check any inactive drives as well.
If there is already a drive using the same name, then change the name of the already existing drive and try to accept the invite again. If you do not have enough privileges to change the drive name, then contact the admin of the drive.