yeah I would not be comfortable with that at all- absolutely not ever while still in a client/therapist relationship. Maaaaaaybe I would consider including a former client in a book club several years after terminating therapy on good terms and with a lot of caveats (including but not limited to a less-heavy topic - idk maybe improving your strategies for executive functioning skills or something )
I can't imagine a circumstance where I would be comfortable having a former client in a group that discussed such a weighty topic even if it had been a long long time. I am curious to know how you plan to clearly delineate psychoeducational group versus therapy-but-not-calling-it-that group for participants given the topic and that it is a workbook which i associate more with therapy -- genuine curiosity. Have you run a similar group before? If so how did you do this and maintain that boundary? Feels like given the overlap with professional role it could be a slippery slope? But I have no experience with book clubs beyond the "BYOB and eat snacks while talking animatedly with interesting people for fun" sense so maybe I'm just having a hard time imagining something between that and therapy group.