In order for a group of skydivers to make a formation of any kind, the most important thing is that everyone have the same fall rate. A person falling at 125mph will not be able to stay with someone falling at only 115mph. If they do manage to grab a hold of each other, the faster faller will put a lot of pressure and tension on the slower faller.
If a skydiver is going faster than the rest of the group, they will probably pass the group rather than joining the formation. This is called going low.
If as a jumper, you have a lot of range in your fall rate, you can slow yourself down and get back up to the formation. In order to do this, however, you have to be falling slower than the rest of the group. So to get back up to a formation that is falling at 115mph, you might have to slow down to 110mph.
As formations build they have a tendency to slow down, so the first people to join the formation might be falling at 125mph, but the later people will approach a formation going only 115mph. If you want to join a formation you have to be falling at the same speed as it.