private group issue: members can't see groups


On my site i created a private group which should allow staff to communicate in a seperate area. 

I invited all the members and they could accept the invitation ... so in my view they all suscribed to the group and are listed as members. But if them members try to enter the group some of them can't see it and some will be redirected to main-page of the whole elgg installation. 

Did I something wrong?

Please help.