I have only ridden regularly with one group, intermittently with other groups, so here is what I know:

A good group will have no problem with you hanging off the back until you are comfortable. They don't want you uncomfortably in their midst any more than you want to be there.

Even 3-4 ft. back, you pick up an advantage. You don't have to go from 6 ft to 4" overnight. Work at your comfort level, and not over it.

Never overlap wheels, and do a minimum of braking to control speed. If you have to overlap, swing far to the left or right. Be 2 ft to the side instead of 4". Get back to your spot without sudden moves.

Never stop pedalling. Soft pedal as needed to not overtake the person in front, or increase your drag.

Most importantly, if you don't feel safe with the group, then self select out of it. Drop yourself. Better to ride safely alone than with company into a crash. As much as I like my primary group, I will opt out if certain riders arrive. What is interesting is that others do the same thing, and I end up with a more comfortable group.