Now I agree wholeheartedly with the many positive things said here and that is where your emphasis should be, but I think it may also be important to feather the brakes a little bit. It is important that experts joining iNat buy into the spirit of the site - to encourage engagement with nature by non-scientists, and produce usable data in the process. Using iNat requires a little compromise, and interpersonal engagement: some professionals may be very fixed in their view of how a particular species must be identified for example, and when joining iNat they will find themselves in a community of knowledgeable people who may have different perspectives, use different characters or have a different threshold of certainty; or they may want everything to fit into their own national taxonomic scheme - but this does not work internationally; or they may bristle at the thought that their vote carries no more weight than others. They must understand that nurturing the community is a primary goal. Once you get that, and you are willing to converse as ‘experts among equals’ exploring nature, everything else follows, and they will gain more as well as giving more as a result, but not every expert will have an approach or temperament that is compatible with the ethos of the site.
Most of all, they need to find it fun! (at least most of the time!) Then others will have fun learning from them, or even disagreeing with them.