I've got a community site with a strict membership criteria: in order to be a member one must be approved by two existing community members. Hence, the question is - what is the best way to implement such workflow. Your advice would be appreciated a lot.
During registration process new users supply the names of already registered community members who can recommend them.
These 2 members receive notification emails with activate/decline links, and make their choice.
In order to activate new user's profile it must be confirmed by both of the specified community members.
Once both of them approved it, user account get activated and user may use the site.
New user is notified of the activation process progress via email, like: 1st accepted/declined your request, 2nd accepted/declined your request, your account is activated/declined.
Until both members accept new user, he has limited access to the site (=special role)
Thanks a lot!

Comments
This is not a how-to, and
This is not a how-to, and nothing related directly to workflow.
Maybe rules.module can help. You'll need the Profile core module to add some fields, like uid1 uid1_code, uid2... Use rules.module to get/fill those values, send email for confirmation... Something like that.
Or write a small module to do that. Won't be complicated with Drupal anyway.