If I were going to a group get together, I would probably have assumed that it was all moms. But if a dad showed up, I wouldn't be uncomfortable at all. Some moms do attend planning on a big gab-fest of gossip. But if this is a newer group, it will just be get to know ya kind of stuff which would easily include you. And hopefully most of the talk will be about the kids anyway. So in a group, I think it shouldn't be a big deal. One on one playdates are harder to get used to, but even then, once you get to know the dad it's not weird at all.
If you have a way to get in touch with the mom who organized, just e-mailing and saying that you plan to attend would ease the element of surprise, assuming you sign the e-mail P.! This is a nice head's up.
If your 'any suggestions' is about how not to be excluded, I would say just be yourself. My daughter is 12, and over the years there have been several stay at home dads that's we've known. A few seemed like they had something to prove, and were pretty condescending. Needless to say, that didn't go over well. The rest fit in just fine and it was just another parent, the focus was not a mom or a dad. And of course, asking about and taking an interest in other children is always nice and endearing!
One last note- if you are uncomfortable, you will probably make others feel uncomfortable. Don't even worry about it.
Glad you are on Mamapedia- I love when the dads answer, it lends a new perspective. Good luck!