I have two questions:
1) Sometimes I need to shoot all kinds of lectures (one or more people are talking and bunch of people is listening, the camera is mostly few meters away from the speakers).
2) I also need to shoot a discussion of employees of some firm, in a way so that they would not be aware of the camera and microphone (I dont mean the mic should be completely hidden, but it shouldnt attract so much attention so that the discussion could be more relaxed and intimate).
What lowbudget sound-recording equipment and techniques could you recommend me for such situations? I'm talking about single-camera shooting, I'm mostly a one man crew.
At this moment I have only Rode NTG2 mic, but I dont know if its the right choice.

Scenario 1 ...

Wireless lapel mic on the speaker. On-camera mic as a backup / crowd mic on your second channel.

Andrew

Just to add to Andrew. Put the Rode on a boom pole and hold it over the group around the table. Aim it and move it as close as you can without being a distraction. The idea is to cut down the boominess of the room audio that you'll get from having the mic too far away.

I do a lot of focus groups and if they're sitting at a table whether it's round oval or rectangle shaped I use a couple of PZMs going to a mixer. I then either run cable from the mixer bacl to my camera or if I need to be moving around the table I'll use my wireless plugon transmitter from the mixer back to the receiver on the camera. I'll also run a hypercaroid (AKG Blueline SE300/CK93) on the camera as a fill in and backup.

If it's an actual seminar type setup then it's a bit different. Mostly the camera isn't going to be moving around the room so I'll pull a feed off the sound board. In all the seminars I've done I've only had 1 where the "soundguy" screwed me and that was the first and last time I worked with him. I think it was the first and last time he worked period.

I do a lot of smaller seminars (actually almost "sales metting type stuff) where I supply the camera, mixer, mics for the speakers and we run the audio to the house speakers which as we all know, suck, BUT I'm not pulling audio from those. I run a (generally) SM58 for the speaker back to the mixer, run to the house wall plug for from the mixer and back to my camera from the mixer. Sound has always been very good except for the poor people sitting out there that have to hear it thru the 8 inch speakers in the ceiling.

Anyway IMO each situation will be a bit different which means you need a little more gear than you have and need to be flexible enough to change the rules for every game.

And what about panel discussions? Lets say 6-8 people... I cant afford so many mics (if the budget is very low). They probably can pass wireless mic between each other, but poeple sometimes forget to take the microphone :)

There is no single "magic microphone" that will solve all your recording situations.

Asking the participants to pass around a single mic is asking for disaster. Dialogs switch back and forth quickly, invariably the mic will be in the wrong place. Additionally, you'll have tons of handling noise which will ruin the recording.

A panel discussion would be the perfect setting for a PZM type of mic. Maybe two or three, though, as Don points out, depending on the size and configuration of the conference table. If everyone is sitting around a 2-meter diameter round table, one would be sufficient (unless the room noise level is terribly high).

There are some relatively inexpensive PZMs out there, designed for conference use... maybe not good enough fidelity for classical music, but more than adequate for speech. Tons of them on eBay, coming out of old educational and corporate conference centers.

Could you recommend me some of them Greg? I have to say I never heard about PZM mics :)

PZM, (Pressure Zone Microphone) is actually a boundary mic. Technically, it's just Crown that calls their boundary mics PZMs.
Believe it or not, the old Radio Shack PZM ( licenced by Crown) were not that bad. In fact there was a mod. to install a 9V in place or the AA and an XLR that even made them better. In fact, I got a couple I would let go of for cheap.

Rick is exactly right, that's why I used the term "PZM type" in my post. The correct generic term is "boundary mic."

And yes, as I recall, the Radio Shack version picked up very well. It was perhaps a bit noisier than the Crown PZM of the same vintage. You might not want to use it on the stage floor to pick up a string quartet, but it would be more than adequate for use on a boardroom table, 1-2 meters from the speakers.

Another example of the boundary type of mic: Beyer MPC65VC. As I said, a lot of companies make this type of mic.