Elderly can be an adjective and a noun. If you use it as a noun, it is plural and requires the word "the" in front of it. It refers to ALL old people as a group, an entire population. It doesn't refer to a small group of old people. as adjective... An elderly man. The elderly women. An elderly group. as a noun... The elderly. (all the old people everywhere) So you need to say... "In all, there were more than 70 elderly people who joined this activity." note: you need to say "... WHO joined this activity."