在嵌套的rails表单上创建多对多关系

I am trying to create Groups, Users and Memberships (the many-to-many relationships) simultaneously. People can add users to the group as they create it and then I want it to route through to a view of the group with all the members. I can get the users to be created and the current user's membership to save to the database. However I'm struggling to get the id for the freshly created User objects in order to save the new memberships.I am trying to create Groups, Users and Members