I have found that it's better to become somewhat acquainted with others before inviting them to WikiTree. I often send them information about WikiTree by email first, and ask them if they would like an invitation. Most people respond positively, but a few have no interest. Basically, I think it's better if you get to know each other a little bit first, which will naturally make the invitation more personal.
Beyond that, I always let them know that WikiTree is a collaborative project, and that reliable information and sources are required for every profile. They should know that I will be ready to help them with any aspect of WikiTree.