solo
The drumset is mainly an ensemble instrument. It and the bass keep time, though there are exceptions...
both
The solo instrument is a violin.
In a band, take jazz for example, a trumpet solo can be a great experience to listen to. You can play it solo but eventually a musical group will use it. I have heard trumpets played solo and they are a magnificent instrument after a required time of study and practice.
An instrument solo in a band is when every one else is quiet but the one instrument that has the solo.
two solo trumpets
The tune Greensleeves can be played by a myriad of instruments. It can be played by a group or on a solo instrument. It can even be sung.
A sideman is a professional musician in a group in which he or she is not an official member. A sideman may tour with the group, perform solo acts or play an instrument with the band.
A concerto is a three-movement form of composition with a solo instrument playing alongside an orchestra. A concerto with many instruments playing in a " solo group " is called a concerto grosso.
The trombone usually plays in a group unless the person who is playing it is doing a solo(dancing/singing/playing an instrument on your own).
The idea of a solo for one instrument out of a group seems to have been quite foreign to medieval music. In fact, composers usually did not assign parts to specific instruments.