Ideally, individually over time, in a friendly relationship. To create a friendship, and then in time to tell my friend of some of the wonderful things that have happened to me. To testify to him because he is my friend. He will convert if it is in him to do so. So, I'm not approaching him just to preach a message and leave, in this scenario, but instead I'm aiming to "love your neighbor as yourself".
Now, there are mostly agnostics and some atheists who are not ardent or religious in their viewpoints, but are about other things in life, but there are some that are more ideological, and you may have to learn in that case to be able to answer some of the deeper questions like "Why is there suffering", which might be in a pointed form like "why do children get cancer" or something like that, and so you'd either have to honestly say "I don't know", or you could learn how to answer that, by reading and learning answers to questions like that. But most are not so ideological, and are very willing to hear about your own experiences, how Christ has changed your life. In contrast if you went to an atheist meeting, that's very likely to be a very militant, ideological group. What then? --
3 “I’m going out to fish,” Simon Peter told them, and they said, “We’ll go with you.” So they went out and got into the boat, but that night they caught nothing.
4 Early in the morning, Jesus stood on the shore, but the disciples did not realize that it was Jesus.
5 He called out to them, “Friends, haven’t you any fish?”
“No,” they answered.
6 He said, “Throw your net on the right side of the boat and you will find some.” When they did, they were unable to haul the net in because of the large number of fish.
In other words, I'd leave a militant religious style meeting of ideological atheism, and instead go make a friend of a non believer....