I've shared underwear before with a couple of different guys.
The first was with Matt, my best friend from junior high and high school. Matt's parents divorced during our freshman year of high school, and because of some complications with his parents he came to live with my family until we graduated. Both me and my younger brother had pretty big bedrooms but since we didn't have an extra guest bedroom, Matt moved into my room where my parents had bought him a twin bed to sleep in. However, there wasn't room for an extra dresser so Matt and I had to share mine as well as my closet. Unfortunately for us, Matt and I both wore literally the exact same size, style, and brand of underwear...white cotton Fruit of the Loom briefs. We were already like brothers and had seen each other in our underwear thousands of times either during sleepovers or during gym/PE class and changing clothes before and after football/baseball practice. As kids and as silly pre-teen boys during our numerous sleepovers, we would often wrestle on the bedroom floor wearing nothing but our briefs. And when we were 12-13, Matt and I would occasionally touch and rub each other's crotch and had even experimented with masturbating and dry humping each other's bottoms while wearing just our tighty whities. I would press my briefed crotch into his bottom and he would do the same to me. It was nothing more than simple boyhood curiousity, but it's not like Matt and I didn't know beforehand what kind of underwear each other wore or what each other looked like naked. Growing up, our housekeeper always washed mine and my brother's clothes together, and with Matt moving in she just added his clothes to the mix without separating them. She'd wash, fold, and put them away in our dresser without trying to figure out whose underwear was whose. After about a week Matt and I had started confusing each other's briefs for our own...I would wear his and he would wear mine, obviously unintentionally. Pretty soon I started to notice on occasion that the briefs I would wear would be somewhat tighter in the crotch/package region. It wasn't too noticeable, but as a guy I could still tell a slight difference. Come to find out I had been wearing Matt's white briefs instead of my own. His were tighter because his privates were quite a bit smaller than mine...so he didn't stretch out or leave as much of an indention in the cotton fabric as I did with mine. When I told Matt what had happened, we just laughed about it and he commented that some of the briefs he'd been wearing had seemed looser in the crotch than his briefs usually were. After trying for a couple of weeks to keep our underwear separate, it became way too complicated to constantly be having to go through our underwear drawer and figure out which pairs belonged to me and which pairs belonged to Matt. So finally without really even talking about it we just started sharing our tighty whities...Matt wore mine and I wore his. We were already too much like brothers to even worry about who's underwear we were putting on. And since we wore the same size in almost everything anyway, in addition to underwear we also wore each other's socks, shoes, jeans, shirts, etc.
The second guy I shared underwear with was my college roommate Seth. We became fast friends living together in the dorm and after our freshman year we got an apartment together. To save money we often washed our clothes together as well, including our socks, undershirts and underwear. I'd always worn plain white cotton briefs so I didn't own any other type of underwear. While Seth also wore exclusively briefs, he underwear collection was different in that he had a mixture of brands and colors of briefs in addition to having quite a lot of pairs of plain white Fruit of the Looms like mine. Looking in his underwear drawer it was obvious that he had way more white briefs than colored briefs, and Seth actually seemed to wear his white briefs pretty often too...3 or 4 times a week I'd estimate...but he also usually changed his underwear two or three times a day. Seth was slightly taller and bigger than me, so his briefs were actually a size larger than what I wore, but when washing/drying and sorting laundry neither of us paid much attention to sizes. All I remember thinking is that the colored briefs were Seth's so those are all I separated. Quite often I would end up with Seth's tighty whities and he would get mine. It got to the point where it happened so often that we started just wearing each other's briefs...the white ones at least. Haha!