How do I know if I have Sanctifying Grace?
Sanctifying Grace is Received in Baptism, lost through mortal sin, and regained with a good confession. A thorough examination of conscience like this one is helpful. A sin committed involving grave matter is only mortal if full knowledge and consent are present.
As stated in the Council of Trent, a person can have a "high degree of moral certainty" of being in a state of grace.
How to Lead a Group
A group can be run by members within the group notifying each within the group when a serious sin has been committed. This option works well if those within the group know each other well enough to be vulnerable with each other. If a person can share their sins to those in a group it becomes more likely he/she can share their sins with a priest. Accountability groups can use code words for "falling" if they like. This can be a fun way to make less awkward the fact that an embarrassing sin has been committed.
A second way a group can be run is by a leader taking the initiative to check in with those within the group on a periodical basis. A group leader could ask on a monthly or semi monthly basis if those within the group have been to confession if having "fallen" into serious sin.
Part of the goal of accountability groups is to avoid sin in the first place but ultimately it is for its members to seek sacramental absolution in a timely manner if a serious sin is committed.