Every state has different regulations about homeschooling. In Maryland, you have two basic choices. You need to 1) report directly to the school district or 2) join a homeschooling umbrella that you report to (and which then is accountable to the state for supervising your homeschooling).
In Harford County, there are Christian umbrellas that are open to all Christians Homeschoolers in the area and there are also umbrellas which are church specific, which means you can join them if you are a member of that church.
There are also co-ops for all different ages of homeschoolers in the area.
Please note that the information on this website is for Christian homeschoolers, if you are looking for umbrellas or groups that do not have any religious requirements, see Many Paths of Natural Learning http://mpnl.org/index.htm or Excelsior Academy http://www.theexcelsioracademy.com/index.html
I hope this information will be helpful!