I mark invasive species by using the Observation fields, as @pisum mentioned! Here’s my methods:
My method for identifying invasive species has been to consult the invasive species lists specific for that state and the surrounding states, as well as the US noxious and invasive species lists. It’s a bit tedious to find all the various lists, but I just bookmark the webpages for my geographic region (southeast US) so I can consult them as needed. Each state/region has different requirements for deciding something is legally invasive, so there’s a lot of ambiguity around introduced species that are little to no threat to native species or are newly-introduced, but that’s another long conversation for a different thread.
My method for marking observations as invasive species is by using the “Observation Fields” feature (in the right hand column on the webpage, right above the top observers list. Unfortunately can’t access these observation fields and other annotations in the iPhone app while I’m in the field). There’s a couple different invasive labels you can add, but my favorite is “Invasive?”. When you choose this option, there’s a dropdown menu with the options of “yes, no, maybe, or unknown”. I find this much more useful than just a straight yes or no, as I can mark plants that seem suspicious (or that are reported invasives in neighboring states but not yet reported in my state) as “maybe”.