In the Christian and Missionary Alliance Church (C&MA) of which I am a member, you would definitely be welcome when attending almost any gathering of church members, whether large or small, plenary or small group.
Our church has "ministries" for almost every age group and for almost every conceivable interest or need (e.g., divorce recovery, 12-step meetings for a variety of addictions, senior citizens, college and career, single parents, grief counselling, church membership classes, growth groups, and many more).
Even if you are simply a disinterested observer, or are there to enhance your social life, you would be welcomed, accepted, and loved.
Does this mean you are therefore free to "sow seeds of discord," or to proselytize, so to speak, for atheism? Of course not. Honest, open, and even heated discussions are encouraged in my church, but as in most every place besides church (except the Jerry Springer show!), good manners are de rigueur.
Does this mean that every C&MA church will be as equally welcoming? Probably not. Despite a shared denominational identity, each local church within the denomination has its own culture, norms, mores, and folkways. Rest assured I am not making excuses for churches in my denomination which are not as accepting of atheists (for example), but I'm just being realistic about regional and cultural differences, whether in the US or abroad.
By the way, I'm familiar with one local church in the Episcopalian denomination which has an Alpha Course for seekers who are interested in finding out more about the Christian faith. Who knows, there you might even meet another nonbeliever of the opposite sex with whom you can strike up a friendship!
In conclusion, all local Christian churches should--ideally--be welcoming of strangers and non-members, but we live in a far from ideal world. Nevertheless, I encourage you to seek out a local church which has an open door policy and welcomes seekers of all stripes, even atheists. When you find one, however, do not expect perfection, just a bunch of imperfect Christians, all of whom are under construction. In other words, be willing to show the same respect, courtesy, and patience they show to you.