Nearly every clinician-led group has some version of it. No socialising between members, or if contact happens, it gets brought back into the room.
Twelve-step fellowships take the opposite view entirely — the relationships outside the meeting are much of the point — which tells you the rule is about what a therapy group is doing rather than about propriety.
Why it exists
Anything said outside can’t be worked with inside. This is the main reason. Two members who talk during the week develop a relationship the group can’t see, and both of them become slightly less available to it — there’s now something being managed.
Alliances distort the room. A pair who are friendly outside will, without intending it, support each other and avoid challenging each other. That’s precisely the material a group is meant to work on.
The difficult thing gets said outside instead of inside. The commonest failure mode. Two people debrief after the session about a third, and the group loses the exact conversation that would have been useful.
And it protects people. Group members disclose a great deal quickly, which produces a feeling of intimacy well ahead of actual knowledge of each other.
What the rule usually is in practice
Rarely absolute. The common formulation is: contact is discouraged, and if it happens it must be reported to the group.
That second clause is doing the work. Not punishment — the group needs to know, because otherwise there’s something in the room that only two people can see.
Some groups are stricter, particularly long-term process groups. Some support groups have no rule at all and actively encourage contact, which is appropriate to what they’re for. Ask at screening rather than assuming.
The objection, taken seriously
If you joined a group because you’re lonely, being told you may not befriend the people in it’s a genuinely difficult thing to hear.
The honest answer: a therapy group isn’t a friendship group, and it isn’t trying to be. What it offers instead is practice — finding out what you do when people get close, with someone present to point it out.
What members consistently report is that friendships elsewhere get easier while they’re in a group. That’s the intended route, and it’s slower and more durable than making friends in the room would have been. More on that.
When the group ends
Most groups lift the restriction afterwards, and plenty of former members stay in touch for years.
If a group is ending and you want to exchange details, raise it in the room rather than in the car park. Doing it openly is, appropriately enough, the last piece of work.