Meet their friends and invite them to your home to meet your family or friends, or to church as appropriate. Attend one of their national cultural events, picnics, or group meetings with them, learning about their culture and values. Share about American customs. Learn songs from each other's countries. Help with English as needed.
Simple conversation is often all that's needed.
Invite them to attend church with you. Attend a Bible study in a small group of other seekers together, such as during one of InterVarsity's monthly international dinners. Talk together about faith. Listen to their needs, concerns and questions. Having a Christian friend or family whose life they can observe and experience is a powerful witness.