Take a look at acs__add_user() for an example of how to add a user as a member of a relational segment (a.k.a subgroup!). That is happening for every new user that registers with an OpenACS site, I believe - they become members of the "Main Site Members" rel-seg. "Main Site Members", in turn, has a membership relation to the "Registered Users" group which it is a segment of. Members of the relseg are automatically members of the surrounding group, I think.
You may want to create a version of acs__add_user() that makes the rel-seg association more general...
Just my $.02
