After writing "solo" over a part intended for one instrumentalist in a section, what's the best way to indicate that the the entire section should again begin playing?
There are a couple of standard conventions for this, depending on the instrumentation you are writing for.
The first, is just writing the word Tutti above the music where the rest of the section is to enter.
The second can be used only if there is a specific number of players in the section. For instance, in an orchestral piece with two clarinets, when the solo ends, a2 is written above where the other player is then suppose to come in. This tells the players that now two people are suppose to play. Similarly, a3 and sometimes a4 are used.