Ambiguous relative clause

Solution 1:

If I were writing this, and my intention was to convey the idea that the participants' friends were told to order drinks, I would write:

"The friends of the participants, who were told to order soft drinks"

However, if my intention was to convey the idea that the participants were the ones who were told to order drinks, I would write:

"The friends of the participants who were told to order soft drinks"

It is still rather ambiguous in print. To avoid this, I might split it into two along the lines of:

"Participants were told to order soft drinks. Their friends ..."

or bring the subject and verb closer together, as in:

The participants' friends were told to order soft drinks