Overpasses for America has floated a proposal to collect dues from its members. Fred Schneider is the state lead for Illinois. See the screenshot image below. It is not clear how this would be enforced in Facebook groups.
A good set of signs and banners for an overpass protest may cost on the order of $300-500. A frequent problem is that participants do not bring signs with large enough letters that it can be easily read from an overpass by passing drivers on the freeway below. A good set of signs attracts participants and also elicits a better response from passing drivers.
Some fear that dues would be used to support management and not to buy materials. To be sure money is paid for signs and materials, it can also be paid by participants to the local organizer, who is making the signs and banners, for specific purchases. Suggest donating a small amount of money to your local organizer, if he/she is making signs. It will make him/her feel appreciated and is not a taxable transaction for the IRS. Or, take up a collection to buy a banner or other material.