"recommend joining" is correct and usable in written English.
You can use it when suggesting someone join a certain group, organization, or social event. For example, "I highly recommend joining the volunteer organization to help clean up the local beach."
And I particularly recommend joining local tasting groups.
I would also recommend joining Twitter and using it as a networking tool.
If the government were to recommend joining, the balance of opinion against joining the euro falls to 8%, a drop of 17 percentage points since September.
There are excellent free resources — pro photographers recommend joining the Flickr group for your specific camera or interest, like landscape photography, for a bottomless well of knowledge.
But before a referendum is held, the government must decide whether or not to recommend joining the euro, following the Treasury's assessment of the chancellor's five economic tests.
I would highly recommend joining your law society on campus, as they held a number of social events in my first term that allowed me to meet fellow coursemates and take some time off.
And polls suggest that, if the political leadership recommended joining NATO, the public would support it.
Anna Gardner, the blogger behind Vivianna Does Makeup, recommends joining weblog publishing tool Blogger as a starting point.
Examples of this include: contacting someone they recommended, joining a professional association, reading an article they suggested, etc.
To switch your home to renewable (or "clean") energy, John Oppermann, a real estate broker and green home specialist in New York City, recommends joining a community solar project.
Marie Benz, editor of the Web site Autorentalguide.com, recommends joining a program like Avis Preferred or the Hertz No. 1 Club, because it allows motorists to skip long lines at the counter.
