Your group has already started and someone wants to join. I feel that this person should wait until your next group starts so that she/he can be in tune, and in time with all group members. I would recommend that the person, any person waiting to start a next group get the audio series and start it, even before the group begins. The audio series can start the person on the right path now, so there is no reason to wait to start it, and the earlier the person starts it, the more they get from group time later. Each member of a group should own the series themselves - also reinforces commitment.
Here at SAI, Dr. Richards runs the local group. In the morning session, the new group members discuss the cognitive therapy sessions in order. In the afternoon, the "returning" group members, members who have been in past groups and continue on - these people join us in the afternoon continue working on their behavioral activities and individual goals.
The morning "new" group members start together. Anyone wishing to join the groups here joins at the beginning of new group start dates - not midway.