Bosses start spawning with 7 teammates, or 6 if you play on challenge levels 2 or 4. If you want smaller teams to see your bosses, you have to put in optional Boss objectives.
Issue 16 made me feel like this.
Warning: This poster likes to play Devil's Advocate.
I set a an ambush and a hostage enemy spawn to both Hard, and ended up with a boss for each in a three-person group, but I think we were are Challenge Level 5 for that. Re-trying it later, though, I got a boss on one spawn, but the second spawn was mostly Lts. <_<
Doing the same arc on CL 1, I just got very large spawns of minions at those two points, but no bosses.
Dungeoncleaners! (ID#125715): Slay the Adventurers! Rescue the Monsters! Return the Treasure!
Peppermint Cat-- Lv50 Mewtant Ice/Eng Bls
I made a custom group that includes generic minions, lieutenants, and bosses. With 3-man invincible team, we're not seeing any bosses spawn, nor are we with 4-man rugged. I'm sure i've seen generic tank and eidolon bosses in normal missions. How many people do you need, and at what difficulty level, in order to get generic bosses to show up in the regular spawns?