Correct. If the user were to find out what was going on they could easily work around it. So don't ever tell anyone. But there's no reason they should find out just because you added them to the MU group.

Do you mean, they could keep there promotion group and just assign them to an additional group? Wouldn't the know if they were assigned to another group?

Perhaps, if you had some system in place that told them, but not by default. Yes, as stated in the instructions this works simply by adding them to the MU group as a secondary, no need to move them there completely.

Because there is no reason to. Therefore Im not going to spend time changing a system that works as intended.

No one is going to know unless (a) You tell them, or (b) they know about this mod and guess (and then they still wont actually know unless you tell them)

I've a trouble with the users in the miserable usergroup, when they try to read a topic they got:

Warning: Cannot modify header information - headers already sent by (output started at [path]/includes/class_bootstrap.php(93) : eval()'d code:23) in [path]/includes/functions.php on line 3742

Then you have something else installed that is clashing.

that are all my plugin:

• Auto HTML Emails
• Automatic Tagger From Content and Title (DISABLED!)
• Extended reputation display
• iTrader
• Make Prefixes Clickable to Filter Forumdisplay
• MARCO1 Advanced Quick Reply And Edit With Smilies
• MARCO1 Quick Admin Tools
• Miserable Users (Actualy disabled for troubles)
• Multiple Login Detector
• Separate Sticky and Normal Threads
• Sevenskins HeaderAvatar
• Usergroup Color Bar
• vBulletin Blog
• vBulletin CMS
• VSa - Check If Already Posted
• VSa - Login To User Account

Clearly you misunderstood. He didn't ask for a list. You need to disable or uninstall all your add-ons then re-enable them one by one to see which one may be conflicting.

I have no idea which of those may cause an issue, you need to do as suggested above.

oh! ok now I check, but it's strange...