The successful execution of knowledge crowdsourcing (KC) tasks requires contributors to possess knowledge or mastery in a specific domain. The need for expert contributors limits the capacity of online crowdsourcing marketplaces to cope with KC tasks. While online social platforms emerge as a viable alternative source of expert contributors, how to successfully invite them remains an open research question. We contribute an experiment in expert contributors invitation where we study the performance of two invitation strategies: one addressed to the individual expert contributors, and one addressed to communities of knowledge. We target reddit, a popular social bookmarking platform, to seek expert contributors in the botany and ornithology domains of knowledge, and to invite them to contribute an artwork annotation KC task. Results provide novel insights on the effectiveness of direct invitations strategies, but show how soliciting collaboration through communities yields, in the context of our experiment, more contributions.