+comset noleave
From Elendor
Syntax: +COMSET/NOLEAVE [<name of set>] Normally, switching in a comset by typing "+COMSET <name of set>" causes you to leave any channel that you are currently on but that is not a member of the comset you are switching in. To instead join every channel in a comset without leaving any channels that you're already on, type "+COMSET/NOLEAVE <name of set>".