It is nothing like as dangerous from a collision viewpoint or informal as you might think.
The standard practice for a glider entering a cloud from below, if no other gliders are apparent in the area, is to announce reg or number, "entering cloud", position reference to a place on the 1:500,000 chart, and alt amsl, on 130.4. If no other glider responds, you know you almost certainly have it to yourself as far as gliders are concerned. It is my experience that GA is normally not in cu clouds on days like that, but if they are, and don't listen out on 130.4, they take the chance often described as big sky, little bullet, but with less safeguard than we try to practise.
Most gaggles of gliders in one thermal have no intention of entering cloud. They are usually intent upon going cross country by the fastest means possible, which rarely involves cloud flying, so they leave the thermal at or below cloudbase.
On the rare occasions when two or more gliders are at or approaching cloudbase and one or more wants to go up into it, the usual thing is for the highest to announce on 130.4 first. The next will generally either go away altogether, or leave at least 500 feet vertical separation before following, after which both call out heights at frequent intervals. It happens so rarely that incidents are almost unknown.
The only one in the UK that I recall was about 30 years ago. Two were in the same cloud, one using 1:250,000 chart for reference to a small village or something for position, the other a 1:500,000 chart and position relative to a different, larger ground feature, both calling out heights as they knew they were in the same part of the country. Eventually one realised they were actually in the same cloud and left, colliding with the other on the way. Both survived, and the BGA then adopted the practice I outlined above.
As I have posted before, power pilots who get excited about the collision risk with gliders are focussing on the least of our and their collision problems. Most power collisions are with the ground, i.e. CFIT. Those with other flying things are mostly with other power. GA power/glider collisions are the least frequent of all - only about 3 in the UK in the last 30 years, at least one of which was in the circuit of a gliding site which the power pilot infringed, and another was between glider and tug from the same site in that site's circuit area.
Most glider collision are with other gliders. Those with powered aircraft included the one mentioned above, i.e. with a glider tug from the same site as the glider. The least frequent type is one with a powered aircraft nothing to do with gliding, i.e. in the same 3 at most, I believe, in 30 years, all in clear air, the most notorious being a Rockwell flying straight into the back of a glider flying straight and level in his 12 o'clock.
Chris N.
Edited to add "from a collision viewpoint" in the first line. Cloud flying by the unskilled is certainly dangerous from a loss of control viewpoint.