Another way of doing that, which can make sense depending on your server architecture/framework of choice, is to repeat the same argument over and over again. Something like this:
/appointments?users=id1&users=id2
In this case I recommend using the parameter name in singular:
/appointments?user=id1&user=id2
This is supported natively by frameworks such as Jersey (for Java). Take a look on this question for more details.