I was added to the portal owners group for full control, however I still lack permissions that other members of this group have. For example I cannot add anyone to a group while they can, also under site settings other portal owners can access Site Collection Features while I cannot.
When using the Check Permissions button I have all the same permissions as the others as well.
Without "full control" you won't be able to create groups in the first place. So you can say that is required to have "full control".
For the standard groups which are created during site creation the owner group (has full control) is the only one who can add users to all groups, so in a way it is required to have full control permissions to add users.
If you are owner of a group with read access or the group is configured so that members can add members you can still add users to a site even if you don't have full control but o give permissions you need someone with full control.
for more info check Sharepoint permission levels
