I'm going to say it - keeping beginners is not something you should be that concerned about. Keeping your regulars is.
Having a good core of regulars will increase your retention rate of beginners which will always show up.
Most people that step into the dojo are searching for something which even they can't define. Some people understand only after a few lessons that the Aikido practiced in your group is not for them, some take longer. You will always loose people - its the nature of the beast.
With that in mind I tailor my class to my regulars which roughly translates into tailoring it for my own views.
I try to integrate beginners as quickly as possible into the class. That means I might pull them aside for a little extra help but generally they do what everyone else does. In fact when beginners do enter the class - its a great opprotunity for everyone to review basics just a little more but just a little.
Those that like the training - like the challenge. I feel no need to spread the word of Aikido. Sure I encourage beginners but Budo is about self training. I would rather make the threshold a little more difficult in the beginning rather than put the time and effort into someone who will probably leave anyway.