When some people join a group, they stay in it for a while. Then suddenly they stop meeting with the group, and people hear nothing else from them, unless they run into them on the street. And when they run into them that person makes an excuse, and say they will be back. But they don't come ever back. Why do people do that? How can dedicated people be found? How does a leader of a group keep membership and growth? Are people allowed to be bounty hunted by their ex-groups?

People come and go from all groups.
Some groups just aren't for some people.

The best advice I can offer is to just keep new members coming.
People who fall into the uninterested category will leave (good for the group) and the people who really care for the cause will be left.
The older your group gets, the more members it will have that actually care about the purpose of the group... just keep new members coming:)

I would guess that it depends partly on what type of group it is and if the person takes it as hounding or feeling welcome. Like when I was going to non denominational churches they would preach, "If you don't see someone in church give them a call and let them know you missed them...." Nobody ever called me and I felt not missed and wished someone would reach out and say they missed me and ask me why I didn't come back. Which I would have told them I have no transportation. Yet I know others that would find somebody calling and asking why weren't you there as a nuisance and use it as an excuse for not going back.
Maybe call and say they were missed and ask why they don't come back. You could even make a survey type thing asking questions like
If they found your group helpful?
If they are considering coming back?
If there is a reason for their not coming back that your group could do to change
Like needing a ride?
If they lost interest in the groups (hobby or whatever you are meeting for)?
What they felt helpful?
What they felt could be improved?