I dont know the law. All I can offer is paragraph 3 of this web page which says a plaintiff in a civil suit must prove the defendant knew of the law. If this is correct, sounds like ACORN has no case.
http://www.rcfp.org/taping/states/maryland.html
Paragraph 4 offers advice that probably applies to O’Keefe and Giles:
“State courts have interpreted the laws to protect communications only when the parties have a reasonable expectation of privacy, and thus, where a person in a private apartment was speaking so loudly that residents of an adjoining apartment could hear without any sound enhancing device, recording without the speaker’s consent did not violate the wiretapping law. Malpas v. Maryland, 695 A.2d 588 (Md. Ct. Spec. App. 1997); see also Benford v. American Broadcasting Co., 649 F. Supp. 9 (D. Md. 1986) (salesman’s presentation in stranger’s home not assumed to carry expectation of privacy).”
The ACORN office was public, and they left the door to the room OPEN. Sounds like they had no expectation of privacy.