I know that Turbine has made recent (and not so recent) strides to make the game more solo friendly. It not my intent to debate that decision at this time. I've merely thought of a few things that could help encourage grouping. Also, not all of these ideas are intended to work in concert.
Please, make comments related to how it would or would not encourage grouping. XP bonuses, TR xp, scaling and the like are hot enough topics and their merits (not related to their impact on grouping) can be discussed elsewhere.
1. Bring back the xp bonus for running a quest under level. Right now the fastest way to get xp is to run normal at the quest level +1. This combined with dungeon scaling is very easy to solo. By encouraging running quests under level, they will be more difficult (by comparison) and more likely to require a group.
2. Remove the group wide xp penalty. A individual penalty of 10% per death or a debt (like the old system) could be given instead. This puts the penalty on the person who died instead of the whole group. This xp penalty is one of the reasons I seldom pug anymore and I've read that I'm not alone on this thought.
Sidebar to #2: Yes its technically a bonus that is not received. But I don't know any gamers who actually look at it this way. AND... didn't Turbine reduce the base xp back when they revamped the system so that completion with the two bonuses is roughly what it was before?
3. Remove dungeon scaling. This goes contrary to making the game more solo friendly, but it would encourage grouping. (Note: I don't think this is where Turbine wants to go with the game, but I felt it was appropriate to list it.)
4. Add more high level content. This higher level content is generally more difficult than the low level content and more often requires groups. (I know even high level Amrath content is solo'd. But its less efficient at high levels to solo, than say, "kobolds new ring leader".) This would also ease some of the pressure many TRs feel to maximize xp, thus they might be more willing to group with someone who might make a quest longer or worth less xp.
5. Flatten the TR xp curves to put more xp required at the lower levels. (This is not the line asking to lower the requirement.) The intent here to allow better grouping with non TR characters. Many people playing multi-TRs are very concerned about xp. This is one option to help aliviate that concern and get them to be more likely to group. (At present, a TR can run a few extra quests or an xp potion and keep pace with a non-TR for several levels. This ends when the levels hit double digits. If the curve was more flat, a TR would work a little harder early on, but have an easier time keeping up at the higher levels.)
6. Reduce the gap between normal xp and TR xp. Either lower TR (you knew it was coming) or raise normal. This is not about it being too difficult to get 4.3 mil. Its not difficult. There is plently of xp in the game, even for multi-TRs. The issue is not being able to stay with a group that is mixed between normal, TR1 and TR2+ characters. Close the gap and grouping would be easier. You would also get to know more people as you level because you would see them longer (as opposed to only grouping once or twice with them because the level gap increases too quickly assuming the same amount of play.)
7. More quests with group mechanics. (I.e. Tear of Dhakan, Xorian Cipher, The Titan, The Abbot.) This is not to say to make everything require six people to stand on buttons at the same time. That can be infuriating as well. Some of the quests with these mechanics are very popular and some just make people mad. This one could be tricky to implement as I believe Turbine does not want to hinder solo play.
8. Make it harder! But less grind. Harder quests require the assistance of other players. Grinds might, don't always (i.e. people solo epics.)