Your friends became relatives without a decision
Your Venus wears Cancer and works out of the 11th house, so friendship here is not a lighter form of relationship. The people in your circle are held with family-level loyalty: remembered, fed, defended, kept through periods when they are difficult and through years when nothing much is happening.
This is why your closest friendships are measured in decades. It is also why the loss of one is a bereavement rather than a change of circumstances, and why you keep several going long after they have stopped giving you anything at all.
Hospitality is how you join a group
Your way into any community is to start feeding it. Within a few months of joining anything you are hosting, catering or organising the part that involves people being looked after, and this is how you convert an acquaintance into a place you belong. It works. Just notice you have taken on a job, and ask for help with it early.
Causes that started with somebody you know
Abstract issues move you far less than a particular person's situation. What you get involved in almost always began with somebody you actually met, and from there it can become years of committed work. That is a perfectly sound route into anything. It does mean your attention is uneven, so look occasionally at what you have never happened to encounter.
Hurt by the group in ways you never mention
Because you are attached at family level, an ordinary social oversight, the gathering you were not told about, the friend who went quiet, lands considerably harder on you than it was meant to. You rarely say anything and it accumulates. Raise the small one at the time. It is embarrassing for an evening and it prevents the silent departure you would otherwise eventually make.
Letting a friendship end is the hard part
You will maintain a connection for years past the point where either of you is getting anything, out of loyalty to who you both were. Some of those are worth keeping on exactly those terms. Others are simply unfinished. Allow one to lapse without a verdict on it, and notice that nothing catastrophic follows.
The friendship that turned into something else
Your relationships often grow out of long acquaintance, which gives them an unusually solid base and makes the transition genuinely risky. Say it carefully and leave the friendship a route back. The ones you have handled well were handled by making it easy to say no, and that remains the whole technique.
You bankroll the group without noticing
Shared costs quietly land on you: the deposit fronted, the ticket bought for somebody short that month, the food nobody split properly. You do not mind and you also have no idea what the annual total is. Work it out once. Not to stop, but so that the generosity is a decision you keep making rather than a habit nobody is examining.
Chosen family, with the same obligations
The distinction between friends and relatives has largely dissolved in your life, and the people in your circle carry family-level claims on you. That is a fine way to live and it needs saying out loud at least once, since obligations that were never stated tend to be assumed by one party and not the other.
You need company and it has to be the right kind
Isolation affects you quickly, and so does the wrong company, which is more draining than being alone. Two people who know you properly, seen regularly, do more for your wellbeing than a full social calendar. Arrange those two as fixed points and let everything else be optional, since the calendar has never once been the thing that helped.
The keeper of the group's memory
You know who fell out with whom, what somebody was going through that year, why a tradition started. Groups need this and yours would be considerably thinner without you. Share the archive occasionally rather than only using it privately, because a circle that hears its own history stays together for reasons everybody can actually name.