I need information/laws dealing with controlled groups and their coverage in a group health insurance plan.

A starting point is section 3(40) of ERISA, 29 U.S.C. 1002(40), which defines a MEWA, which may be what you're trying to avoid (or determine). The definition will send you to section 4001 (29 U.S.C. 1301), where there are regs, which, in turn, will send you to IRS regs. Note, however, that 3(40) uses 25% as the important number for determining common control. All control group members can be included in a single health plan without forming a MEWA.

33fan is correct about MEWAs. The DOL has issued a number of notices this year dealing with MEWAs that are extremely helpful. I believe they are on the DOL's or PWBA's website and it is critical that you read them. If you do have a MEWA there are new filing requirements Form M-1.