Q:

is it legal to charge for wordpress multisite?

I want to create a wordpress multisite and charge my members a monthly fee to use it. Is this okay under the license agreement?

A:

Yes. The GNU General Public License explicitly allows for charging for redistribution (which you're not actually doing in this case):

You may charge a fee for the physical act of transferring a copy...

...and explicitly allows for unlimited usage rights:

Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted...

Note that there are already several commercial services based on providing WordPress as a service, including Automattic (wordpress.com), WPEngine, and others.

