private registration?

I am planning a private elgg site and am wondering if there are different ways to set up the registration. The plan is to have people join a wordpress site and to open registration to the Elgg site only to those who join the wordpress installation.  The way I understand Elgg at this point is that you can make it private but you can either allow registration for visitors, or not.  This would leave us with a private site but the registration would be open to anyone.  We could alternatively close visitor registrations and manually register those who join.  That seems a bit onerous.  

Are there alternatives that might help with this?  I wonder about having the registration page for elgg be restricted somehow, possibly in a password protected area? I am hoping there may be a simpler solution and very much appreciate hearing from those of you with the experience and know how to understand other possible options.

Many thanks for your input.  Tom