That’s an interesting question, JP59
The average roll rate for a 15m glider is about 4-4.5 seconds from 45 degrees left to 45 degrees right at about 50kts.
The average thermalling speed is about 50kts, and most competent glider pilots thermal with somewhere between 30 and 45 degrees angle of bank.
As regards to manoeuvrability and proximity in thermals, gliders established in a thermal will generally all be ascending in the same parcel of rising air, and therefore vertical separation would remain reasonably constant assuming that they are all climbing at the same rate and same efficiency. In practice there are slight variations and some pilots can climb faster than others while maintaining horizontal separation.
Horizontal separation is slightly different and requires an understanding of the convergace/divergace speeds and what to do if one or the other speeds up or slows down.
An overarching principle of constant lookout, situational awareness and airmanship ensures safe flying in a thermal, and gliders are sufficiently agile/ manoeuvrable at these speeds to ensure separation is relatively easily maintained.
There are well defined principles for joining, flying in and leaving a thermal to ensure that everybody can (in most cases) anticipate another gliders actions.
What really screws this up, if somebody, be it a glider or powered aircraft, does not adhere to these principles.
It takes significant concentration to fly in a tightly formed gaggle, especially in a competition where there could be upwards of 30 gliders in the same thermal. The levels of concentration tend to be focused on maintaining safe levels of separation... perhaps in this situation there is less focus on the PA-28 cruising in towards the gaggle at 90kts from the southeast!!
Does this help?