Yichud #4

The mishna says that one woman is permitted to seclude herself with two men provided that they are morally chaste [not prutzim]. However if they are indecent people [prutzim] even two or more men with one woman is forbidden. This means practically that if two workmen come over to fix the air conditioning and odds are that they are morally unrestrained, a woman may not seclude herself with them.

There is an interesting opinion of the Rambam who says that we ALL have the status of prutzim and therefore a woman may not seclude herself with 2 men or more. This is based on the gemara that relates that Rav maintained that both he and Rav Yehudah have the status of prutzim and non-prutzim would only be someone like Rebbe Chanina bar Papi and his friends who were really holy.

Two men and two woman is the subject of a dispute. Three and three is definitely cool [permitted].