His disciples, who were a known group
"We" also could have also included Jesus' cousin, John the Baptist because they were somewhat similar in their messages and related in their public ministries.
The hermeneutic here would relate to a presumed context. The meaning of "we" is assumed rather than explained. So, whatever group of people that "we" meant, Nicodemus already knew about them enough that they needed no introduction.
Jesus was already starting to be somewhat of a "thing" at that point, which was why Nicodemus both wanted to meet with him and to hold the meeting [semi-secretly] at night.
What we learn from this
What we take away is the point of hermeneutics.
"We" may stand out to us as a non-involved audience trying to understand the context by reading a story set within that context. But, "we" was not the main focus of Nicodemus or Jesus because, as already mentioned, the "who" behind the "we" was already presumed.
This message that "we" testified about was a message from what they had seen and known intimately. "They" had a message about personal experience, but Nicodemus—at that early point—only had theoretical knowledge. We could compare this to Job's words at the end of his story...
Job 42:5 (NASB)
“I have heard of You by the hearing of the ear;
But now my eye sees You;
On some level, Jesus is making an allusion to Job because both Nicodemus and Jesus would have been quite familiar with Job having first "heard about God", but later had actually "seen God". Consider Jesus' word for "know" (οἶδα/oida as opposed to γινώσκω/ginóskó), the way Jesus knows the Father. This commentator explains its usage elsewhere, including the Gospel of John...
Ginosko and Oida | A Day of Small Things (emphasis added)
The difference between the two words is illustrated in John 8.55, ‘Ye know [γινώσκω/ ginóskó/Strong 1097] Him not, but I know[οἶδα/oida/Strong 1492]Him.’ Again, in John 13.7, ‘What I do thou dost not know [οἶδα/oida/Strong 1492]now, but thou shalt know [γινώσκω/ ginóskó/Strong 1097] hereafter.’ And finally in Heb 8.11, ‘They shall not teach . . . saying, Know [γινώσκω/ ginóskó/Strong 1097]) the Lord; because all shall know [οἶδα/oida/Strong 1492]me.’ The word [οἶδα/oida/Strong 1492]is used of Christ as knowing the Father, and as knowing the hypocrisy of the scribes and Pharisees, of Paul’s knowledge of ‘a man in Christ,’ (2 Cor 2:12) and of the Christian’s knowledge that he has eternal life.
Jesus and the "we" he mentions had a deep, personal knowledge from real experience. This knowledge affected who they were, what they did, and it made them so well-known for the impact it had on their character that it drew the attention of Pharisees like Nicodemus who wanted to know more because their knowledge of the same was only theoretical.