The usual framing is safety, and that is mostly the wrong frame. Here is what actually differs.
Money, which is the honest headline
Most trek costs are per-trip, not per-person: the guide, the vehicle to the trailhead, permits, and often the cook and the kitchen tent.
Split four ways, those numbers look very different from split one way. A solo trekker frequently pays a significant premium or is asked to join a fixed departure to make the economics work.
If budget is the binding constraint, going with friends is simply cheaper, and it is not close.
Solo almost never means alone
This is the part that reframes the comparison.
On most Indian Himalayan routes, solo does not mean unaccompanied. Several states require a registered guide for particular areas, Forest Department permits are commonly issued through operators, and the practical default is that a solo trekker walks with a local guide.
So the real comparison is not "with friends" against "by yourself". It is "with friends and a guide" against "with a guide". That difference is far smaller than the word solo implies, and it is why guide availability matters more than anything else when you are on your own. Our going solo list is filtered on exactly that: nothing graded challenging, and the destinations with the deepest bench of verified local experts.
Pace is the daily friction
A group moves at the pace of its slowest member on the uphill and its most cautious on the downhill. Over five days that compounds, and the gap between the fittest and least fit person in a group of friends is usually wider than anyone estimated in the planning WhatsApp group.
Solo, you walk your own pace all day. For some people that is the entire argument.
The decision problem nobody plans for
This is the one worth thinking hardest about.
Every trek involves judgement calls: the weather has closed in, someone has a headache at 4,000 m, the pass day looks marginal. Turning back is the correct answer more often than people expect.
In a group of friends, nobody wants to be the one who calls it. Social pressure runs precisely against the safe decision, and it runs hardest on the person who most needs to make it. Solo, with a guide, that conversation is between two people and the guide's judgement carries professional weight.
Groups are safer in the obvious ways: someone can go for help, someone notices you are struggling. But they are worse at deciding to stop, and altitude illness is a condition where deciding to stop is the entire treatment.
Agree before you start that anyone can call a turnaround without arguing about it. It costs nothing and it is the single most useful thing a group of friends can do.
Which routes suit which
The routes genuinely differ, and it is not only about difficulty.
Better with friends are the longer, harder ones where shared effort is the point and the per-head economics improve most. Goecha La at eleven days, Hampta Pass as a crossing, the Chadar. Our trips with friends list is filtered to five days or more at moderate or challenging grade for that reason.
Better solo are the shorter, non-technical routes in places with plenty of guides, where you can set your own pace and arrange things at short notice.
The safety question, briefly
Solo trekking is legal and normal across most of the Indian Himalaya, with the caveats above about guides and permits in specific areas. The genuine risks of going alone are an unwitnessed injury and no second opinion on a judgement call, and both are largely answered by walking with a guide, which you were probably going to do anyway.
Register your route, carry a way to call for help, and tell someone your plan. That advice is identical for groups.