I need a command to add a user to a group. Some users have more than one group but the "/lp user {name] parent set hero" for example will change the group since it is parent command. I need a command that will just add the player to another group. I can do this in the webgui but I need the command since it is for buycraft.

Try parent add.
Check the Command Usage page for further reference.

Trying it now

Works perfectly many thanks!!