It's just like fog, some thick, some thin - same stuff, of course.
Visibility can vary quite a bit, so sometimes form in cloud is straightforward and other times you're staring intently and seeing bits of the leading aircraft fade in and out of view, hoping it doesn't disappear entirely.
On that note, you have a lost contact procedure which will involve some kind of positive break away (a turn, climb or whatever's appropriate for your position in the formation) and then heading / height separation established so you can operate singly.
The formation flying procedure is the same in that you look at the same cues and do the same things with your hands and feet, but wingmen can easily get a case of 'the leans', particularly if punching in and out of cloud and getting glimpses of the ground in your peripheral vision.
Aircraft with radar / useful air-to-air aids can use that to be close to others in cloud, but I've never been lucky enough to have gadgetry that could do it so I can't comment on the specifics!