Here are some videos on rhythm and strumming that I find helpful.
First, a video by mandolin player Don Julin showing three easy rhythms with the tune Whiskey Before Breakfast used as an example.
Next, a couple strum pattern videos by tenor banjoist Steve Caddick. (Steve's tenor banjo is tuned CGDA. If you tune your tenor banjo/mandolin GDAE then just transpose everything by a 5th. So for example a C7 chord on a CGDA tenor banjo would be a G7 chord on a GDAE banjo.)
In the 2nd Steve Caddick video he demonstrates what he calls the "Do Wack-A-Do" strum.
While I'm at it, here's a Steve Caddick video on some hand positions for tenor banjo (applicable to any 4-string "tuned in 5ths" instrument like mandolin, mandola, et cetera). Again, since I tune my banjo GDAE I know that when he refers to a G-seven-five chord that means it's a D-seven-five for me.
And in addition to those videos here are some links you may find helpful on this topic. Such as this page with with info on basic rhythm banjo. (Once on this page you'll find that there are additional accompaniment pages that might be of interest.
Less applicable to chording, but worth noting, is this cool article by Mike Mulready on Irish Tenor Banjo style. I'm primarily linking to it here so that I don't forget about it and can return to it later when I have time to look at it more in depth. I'm particularly interested in checking out his examples of Straight vs. Irish Style versions of session tunes The Silver Spear and the Irish Washerwoman.
Lastly - this link will take you to a series of archive.org videos by tenor banjo player Pat Costello. You may have to weed through a lot of basics to get to the helpful stuff, but it's well worth it. Pat has an easy-going, easy to grasp, method of explanation. He's coming from a folk perspective that a lot of us amateur musicians can relate to. He's not trying to be a Buddy Wachter, he's just showing you the fundamentals so that you can get more enjoyment out of playing your chosen instrument. Pat Costello also seems to have lots of videos on You Tube, but I haven't had a chance to look through these yet. Pat and his son Patrick Costello also maintain a weblog at dailyfrail.com. They update it frequently...hence the name daily I guess. Perhaps worth checking out from time to time.