number of invitation fopr private group

I created a private group. One personn already joined the group. I invited a third member. When he received the invitation , and try to accept it, the following message appears: " this group has reach its number limit. The group owner need to upgrade the account, for you to join this group". It is clearly presented that it is allow to have 3 members per group. Could let me know what is teh procedure to do ? Thanks