Riding with bikes...

Quite a few people on the forum have ridden event rides with bikes; both long road rides and mountain bike events. The one you linked to looks like a good choice; I’d register for a distance I was sure I could handle.

Group road rides starting from LBSs are a different story. These rides are usually at a faster pace than a unicycle could hope to maintain. I would focus on training with solo rides and not worry about riding with bikes before the event. Try to put together some routes that are similar to what the event course will cover. If the event is close enough for you to pre-ride the route, even better.

Here are a few event threads:

Riding in a non-uni race

Seattle To Portland

Black Fly Challenge:

Singlespeed-a-polooza:

There’s a bunch out there.