The collab didn't take place in its own little pocket dimension.  Real time passed during the duel that anyone can use.  Anyone in the entire roleplay could have found them, watched every blow and witnessed every power, and can now interact with them.  There is absolutely no 'cutting out' people watching.  And participating doesn't mean leaping in for no reason to start whaling on someone they know nothing about; with a high-profile fight like this, someone in the vicinity could have easily decided to dodge some mashed potatoes or get hit by cracker crumbs.  The only thing that having a collab post did was make it so nobody could directly interact with Juri or DIO.

The fight's purpose was to establish the character of two separate individuals, begin a relationship between them, and start a subplot based around Juri developing her own stand.  If I did things simply for their own sake, I'd belong in the Free section.  Oh, and this roleplay's been going for nearly a month.

Next time there's the chance of a collab, however, I'll make an announcement post about it on the OOC, and anyone who wants in can come in.  That's the single way it could be easier for other people to get involved.