It's not when they join. At least not in most communities.
Many of those that register don't have the intention of ever participating. You're wasting a lot of your time for minimal results.
Far better to welcome someone shortly after they've made their first contribution.
These people are on the edge between being a regular participant or lurking.
Drop them a note and make a comment about the impact of their first post. Suggest another discussion you might like their opinion on. It's a small, direct, tweak that has a big impact. Try it.
On October 29th to 30th, the world's top 250 community professionals are going to SPRINT in San Francisco. Will you be one of them?