Will they have the app already installed? My experience with adults downloading the app during a presentation is that there are varying tech abilities. Common friction points are confirming the account email, ensuring location is turned on for the app, and working with a wide variety of phone and platform builds.
I would start first with downloading the app before you do any sort of presentation, even as people are coming in. Be prepared to spend about 10 minutes getting everyone squared away. Have them write their user names on flip chart paper or a board.
Once everyone has the app installed, I would first do a short overview of iNat and the data (10 minutes). If you wanted to add some interactivity, you can make iNat or iNot (make a copy in edit view to tailor to your place) a mentimeter activity.
Then I would tell them about Seek b/c often retirees are looking for things to do with grandchildren.
Then review tips and tricks for good iNat photography and let them loose. Meanwhile you or some other staff member quick cobble together a project filtered by date and user names. Have them come back at about 10 minutes to the end of the program and show them the project.
I’ve found that adults usually don’t need more to be engaged. There’s a lot going on with just figuring out the app, finding things to take pictures of, adding an ID and all the things. Creating an additional challenge in my opinion is an advanced beginner activity.