These are worshippers of a radiation god which they reverently call the “Radiant Divine Glory”. These cultists are those with high radiation resistance and many are mutated, as initiation requires that they spend a week in a radioactive area. Radioactivists are often seen traveling the desolate areas of Gamma World, searching for new “holy lands”.
In game this secret society is often abused by players for the extra mutations at character generation. It has been suggested on many forums that the GM should enforce appropriate penalties for PC's that do this, although this is left to GM discretion as each case is different.