As a general rule: anything which is clearly academically related can go on your CV. As you say, there are certain things that are required or nearly required on your CV, but even these vary by discipline and sometimes people in the same discipline have divergent ideas. (For instance, every once in a while I find a CV which doesn't list the candidate's undergraduate institution, or lists it without indicating the years that they spent there. I would have thought that was strictly required. I guess I was wrong.)
What to fight in adding "academically related" information to your CV is clear: if you put in too much routine stuff, you'll get a bloated mess of a CV that few people will actually read. Drawbacks of a lengthy CV can be mitigated by very careful and clear organization. Nevertheless, you can compare CVs of two absolutely cognate people -- e.g. who attended undergrad and grad at exactly the same time and arrived at their current institution at the same time and with the same seniority -- and you'll find significant variations, including up to at least 1/3 in the total length. It's really your choice how much detail to put on your CV, and I would be skeptical of anyone who tells you too stridently what you must and must not do.
Having said that: yes, very early career academics (especially students) really struggle to have a CV that doesn't look unhelpfully skimpy. Conference work sounds like a great thing to put on a junior person's CV -- and in fact, even on a senior person's, although a more senior person might be more selective about the kind of conference work that goes on your CV.
One tip: "conference volunteering" might be an undersell. Most conference organizing is unpaid, hence is volunteer work. "Conference assistant" sounds like your work was really on the level of secretarial work / technical support. If that is an accurate description of what you did, great. If you do such things more than once or twice, you should think about asking to be let in on some of the more content-related aspects of the work. For instance, if you had a say -- however small -- in choosing or recruiting the speakers or attendees, then I would go for something like "co-organizer" rather than "assistant".