The episode did not show exactly how the band members knew where to meet Squidward at for rehearsal, so we can only make some guesses. One is that Squidward gave the address to either SpongeBob, Patrick or Mr. Krabs, and one of them passed the information to their friends. Another possibility is that Squidward had to create another flier with the address on it (that can be the case since all the members had arrived at band practice before Squidward did).