How to move many groups into a topic

[hput]

Sorry for the dimwit question, but I've been withpit gnus running
for a good while and as always for us older folks... I've forgotten
an awful lot I once knew about gnus operations.

I like topic mode where I keep related groups in topics in gnus 't'
mode.

I've let things go so long that I have an awful lot of nnml groups that
need to put in to various topics.

How does one mass move them into that topic... some king of use of
process mark? or what?  Also how does one indent a top under another
more general topic... I recal actually using the tab key to do that.
but something more must be required as it has no affect on a topic
currently. 

But mostly I need to some how move bunches of nnml groups into a few topics:

Any ideas?  Or is it just done one by one?