There are no restictions (e.g. with regard to your affiliation, expertise, or being a governmental employee) for the people who would like to attend a Committee meeting. Most host countries of a Committee have their Codex Contact Point made responsible for dealing with the registrations for a forthcoming meeting. The Codex Contact Point of your country will receive invitations for a meeting and is finally responsible for selecting participants. By its selection, the Codex Contact Point can take some national restrictions or agreements into consideration, but the Commission and the Committees have no involvment with these.
Members and Observers can be recognized in the plenary rooms by their name signs. If you are not present on behalf of a Member or Observer you can only participate as a "guest". Guests are "backbenchers", they can not join the discussions, and are not seated in the plenary room between Members or Observers.
The Chair of the Committee will consider each participant as a representative of the Codex Member or Observer, and there is no check by the host country with regard to the attendees specifications. So, if you make an oral or written statement in the meeting, it will be considered as a formal position of the Member or Observer.
So, you are advised to check the the meetings schedule to find out when and where a meeting is taking place. Then you should contact your Codex Contact Point to express your interest in one or more meetings. The contact information of the Codex Contact Point is to be found in the details of your Member country. The Codex Contact Point will consider your request (taking into account possible national agreements regarding participation) and send the necessary details to the host country for registration.
One should be aware, that the Committee host country does not take care of travels and accomodation. It will provide some social programs free for all attendees, but for the rest of the days attendees should take care of all costs of lodging and meals themselves. Sometimes the host will inform you about lodging with special meeting rates but it is yours to decide to use these or not. There is however one exception; the FAO-WHO Codex Trust Fund can be used for (financial) support for attending Committee meetings. More information, e.g. how to apply for support, can be found elsewhere.