I have been soloing for about 20yrs every since I first started playing mmo's when Ultima Online came out. I grouped here and there and joined guilds. Guilds are not the end all to beat all and everyone knows how many problems comes with guilds. Here is a thought for ESO. To really get players to group together and please try this. Give out GOLD items resources. I think these are them, so please don't flame me if they are not. Tempering Alloy, Chromium Plating, Rosin, and Dreugh Wax. But the idea is something great for grouping like 10 or 20 PUG groups. Like don't put it for the daily dungeon because we are already getting XP for that or the daily battlegrounds. BUT, if eso would give out these rewards or add to the Tamerial Tomes. but I like gold resource rewards for odd grouping stuff. Like it's hard to get groups on off times for "Vet dungeons, Trials, Infinite Archives, specific dungeons (grinding to get sets) etc. etc.
Don't even say join a guild, because eso guilds really need help. They are just a place to use members for traveling.
Anyway. please consider this, because there are alot of people who don't mind grouping when they need to get things and are unable to.