Is it legal for probation to make me do sex offender groups when the charge happened over ten years ago?

Is it legal?  Sure.  Is it necessary?  Maybe not.  Probation is given wide latitude in what conditions they request for offenders.  They have to substantiate those requests to a judge who then orders them.  Again probation and parole are not rights, they are jail alternatives.  If you don't believe the conditions are necessary you could ask the court to reconsider but you should talk to local counsel before doing so.