I would ask your MOH to organise a part of the hens day/night that your junior BM can also attend - like something pampering, or something like that, so she doesn't feel left out.
Also, she'll feel special if she has her make up done with you as well, even if it's just some lippie/gloss and colour on her cheeks.
For a job for her, what about making her responsible for one part of the day? Like she can mind your lippie and tissues or something, and be responsible for looking after that. Or some other job on the day that she is 'in charge' of - like making sure every has their flowers, or straightening your dress etc.
For other planning jobs, perhaps tell her you need her help with invites or something, and have her over to your place one afternoon - she can put the invites in envelopes and stick stamps on. Or have her help tie ribbons onto the bomboniere or whatever you're doing.
I hope that helps