If the characters are family, using the family gathering is a good way to introduce them. Give them doing or saying something that helps your readers to remember them. Or they might have an unusual physical feature or mannerism, or wears something different or uses a cane. If they are not related, they could be at a social gathering such as a dance, a resort or a school reunion. Another way to help readers, is to list them on the page before the story starts, with a brief description as how they relate to main character. Authors such as Agatha Christie used this technique sometimes.