There are a number of reasons why this can happen, here are a few common issues to be aware of:
  1. If the group is an interest opt-in group, API calls will not add the constituent to the group. To add a constituent to an interest opt-in group you should instead subscribe them to the corresponding interest, then they will be automatically added to the interest opt-in group.
  2. You can only add a constituent to a group when the API call is made on behalf of an admin (an admin account is used for authentication). Since users cannot add themselves to groups, this functionality will not work if the authentication is for a non-admin user account.
  3. You cannot use the add_group_ids parameter to add a constituent to any of the pre-defined groups (those with group IDs less than 1000).
  4. You cannot use the add_group_ids parameter to add someone to an administrator security group
  5. The administrator account used to authenticate the API call must have the proper security access to add constituents to groups. When editing permissions for an admin security group, the relevant permission type is "Group Administration". Note that the "Use Convio Groups API" permission will only allow the admin to call server API methods for groups.