Yes, but you are also supposed to witness to the lost. This is not witnessing to them, and is in fact directly accomodating behavior which does not lead them in that direction. The comments in the article indicate they are treating this as two social clubs holding joint events - like if your local Kiwanis Club and Lions Club decided to hold a joint events or use each one another’s facilities to help one another out.
2 Corinthians 6:14 (New King James Version)
14 Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?