If everyone is going to throw their opinions out there, I might as well too:
First, try to find a drummer. It'll making jamming feel stronger. The drummer is the life of the songs.
That aside, a lot of these songs can be half-assed. There's a
video tutorial on Fuwa Fuwa time back in 2009 when it was released and it's as basic as you can get.
As basic as you can get is probably Let's Go. There's nothing really too hard about it, there's not even a solo for any instrument, mainly because this song was focused to have a vocal dominance. Pre-chorus is can be killer for your guitarist though.
I see some people are saying Curry Nochi Rice, but it's tricky if you can't play up to speed. Go Go Maniac is played at about 250BPM (Beats per minute) while curry is played at 258BPM. If I'm not mistaken it's the fastest song in the series.
Gohan wa Okazu is pretty straight forward as well. The bass doesn't do anything too irregular, and if the solo is hard just play the rhythm guitar. The keyboard is split into four parts, so just pick and choose and try to find out what works best.
Honey Sweet Tea Time is pretty straight forward as well. The bass starts to walk and stop a bit and it creates irregular patterns sometimes. If you want to do this one, practice, practice, practice. It'll take time to memorize which part is played where. This is another vocal driven song, and the end solo is really easy.
Tenshi ni Fureta Yo is a pretty song. From how I look at it, it's not a hard song. The only thing is that the bass walks sometimes, but it's really not that different from learning a solo, I always think of it like a solo spread through out the song. The rhythm is also a little more complex than other K-ON! songs, especially in the chorus.
U&I is another pretty song. Keybaordist may have a little trouble with the interlude, but the fact that you have both a keyboard and a bass make that work wonderfully. Really simple song really. I suggest you should start with this one because of the difficulty and the fact that it's one of the most notable songs in S2.
Don't Say Lazy. Just "don't get lazy" with this song. It's simple and straight forward, but the breakdown may throw you off.
Fude Pen Ball Pen is really simple as well. The Palm-muting can be a real challenge though. Tell your guitarist to really focus on the rhythm, either that or tell him/her to play the lead. It's much easier.
Sweet Bitter Beauty Song is pretty much played with half notes and whole notes. If it's playing neither, it's a simple lick. It can get really repetitive which makes it super easy. The only hard part is the last section of the solo.
Character songs aren't easy, just going to throw that out there.
So pretty much, I just gave you a master list of all the songs that are easiest to play. Remember though, because they're easy, doesn't mean you should only practice when you get together. A good band learns the song in advance and usually more than one at a time. I suggest you and your friends take about 3-5 songs, and play them. Don't sit on one, get it perfect, then move on. I guarantee that you'll get sick of the song and never touch it again after you get it perfect. You can also dumb it down for some of the parts and don't be afraid to. It's a cover and it doesn't have to sound like the original (but maybe at least recognizable).
Also, don't worry about not getting it to a presentable fashion even after a long time (in the anime, the girls are considered terrible ["You guys aren't good at all!" - Yui] ["You guys are terrible" - Azusa]), just have fun with it and hopefully you all get better and better.