[functions_config] Fixed problem with incorrect post_ID variable being used. Caused post meta to not be saved.
[single-kzm_group] Added link to 'Join Group' page.
[functions_groups] Added a AddUserToGroup() method to add users to groups. Adds the user and group details to the 'user_has_group' table.
[mxpress_index] Added use of template_group_join.php template for 'Join Group' page.
[template_group_join] Created to handle joining mxit users to groups.