What constitutes being mafrish from the tzibbur? The Tur ends off the siman by saying if you don't fast together with the community, you're mafrish from the tzibbur and won't join them in their eventual rejoicing.
The MB comments that being mafrish from the tzibbur means not praying, fasting, or giving tzedakah when what you do can make a difference.

Does that mean simply disagreeing with the tzibbur, let's say not joining in a rally in support of a community cause is not being mafrish from the tzibbur?

what about not attending the shul fundraiser banquet?

Right, although that could be an example of where you are withholding tzedakah, which is being mafrish. On the other hand, if you donate the money, but just don't attend, maybe that's not being mafrish.

So whenever shul is asking for money it is ASSUR not to give?

Maybe. Well, not necessarily assur, but it's being mafrish from the tzibbur perhaps if you have the ability to give and there's a real need [a REAL need, not that the shul wants new carpeting in the social hall; starving widows and orphans let's say] and you sit on your hands and refuse to help...maybe, yeah.

What does it mean to make a difference in this context?

I don't know. But look at the last Mishnah Berurah in this siman - he seems to qualify being mafrish from the tzibbur and limits it to when your tzedakah, tefillot, or fasting will make a difference.
I'm guessing if you can only donate a token amount, and you don't, then that's not being mafrish since it wouldn't have made a big difference anyway.

thanks.