Allow arrays in add_to_group

In the documentation it states that you can pass in an array of ids to add a user to these groups, but this currently does not work. I've updated the function now to allow that.

The function will return FALSE if any of the IDs passed are invalid, if all IDs are valid the function will then return the number of groups added successfully.
It's probably not worth having that check at all really.  FK constraints would handle it and it's a performance hit as is.