First of all, roundtables are just groups of people who have grown to be friends on the air. Sometimes they know each other in person, but often do not. So one thing a person can do is listen to a specific roundtable for a few nights and see if it is an interesting group. If so, check into it sometime, and see if you can contribute something to it. Don't just check in for a 'radio check' though. Have something to say on the current subject.
Nets are a bit more formalized, and often are set up on 75 meters to handle regional traffic, though they rarely do. Usually they have a time when a guess can check in, as the net is wrapping up, but unless you have a specific reason to fit it, it is probably a waste of time. You may get a signal report, but not much else. If that.
If you can find a clear frequency, and be sure it is clear, calling CQ is completely appropriate on 75 meters. Just try really hard to avoid interfering with someone already there. You might be surprised. I rarely call CQ anywhere, but I hear people on 75, as well as on 40, call CQ and get answers.
On 75, though, you have to battle the conditions. In the early evening short range contacts up to 200 miles are the norm, but by 8 pm local, that has expanded to 500 miles. Plenty of noise, not just atmospheric, but local power line, electronic gadgets, and the like, can affect how well you hear on 75. That is not as bad on 40.
Remember, too, a lot of those 75 meter folks are funning legal power or near it. But keep in mind, only 3db separates your 100 watt signal from his 1000 wat signal. So if he is listening where you are calling, he may well respond ot you. If you have monitored 75 much, though, you know it is a noisy band. Not QRM but noise in general.
It should improve in late November, at least a bit. Over my man years in this hobby I have used 75 for local contacts almost exclusively. I have 'accidentally' worked some DX on it, as well as on 80 CW, but most often I have found I use 75 meters for up to 200 miles, and when it goes long after about 8 pm, I just move to 40 or 30.
Understand the band, understand its propagation, and you will see why it is so popular for nets and round tables. A round table can hang in there for several hours without propagation becoming a serious problem.
So give it a try, but realize what it does best before you do.
Ed, CHOP, W5HTW - Novice 1956, General, 1957, Advanced, 1968, Extra, 1969. Keep the [B][U]amateur[/U][/B] in amateur radio, keep the pros, and Part 90, out of it.