Get thoughly tested.
Read up about STI's and other virus's first. Then see if you want to go down that path.
My personal story is that we did the whole STI check thing and only had inter group sex. What we didn't know was that they don't test for Herpes unless you have visable blisters.
Long story short, I now have to tell people I'm positive for HSV1 (commonly causes coldsores, but can cause genital herpes). Finding that out was the kiss of death for our group, while I know intellectually that at least this isn't that bad, emotionally it stirred up and still stirs up alot of negative feeling.
Go into it with all the information and with open eyes, at the end of the day you just need to work out what your group is comfortable with.