I found this when my DS was an "only" chid,I was his playmate at home.
At playgroup have you tried to leave him? I have seen, and had friends, whose children are very upset to be left,but mostly the child either stops crying as soon as you are not in view,or the playgroup leaders start to interest them in things,to distract them.
My friend's DS was very "clingy" (awful term)and he (and she) was distraught each time she left him at playgroup,she was in a real dilemma as to whether she was doing more harm than good,our "baby group" of 3 yr olds Mums,all agreed she needed to percivere(sp?)so she did and now less than 2 months later he is loving it,and is happy to be left.
IME alot of children don't like to be separated from their mums at this age,but it is a stage they all have to go through,and he will come through it,but I would suggest you do leave him,the Playgroup people would contact you if they thought he was too distressed anyway.
If you want your child to build relationships with other children and adults then you have to give him the opportunities to do it,whilst you are not with him.It is very hard to walk away from your DS when he is upset,but Playgroup is used to this reaction,and will have "strategies" to help you and your DS.