I find there is a lacking in a good word to define it as a plural.
Discussion
Ah, that's an interesting point. In most cases, "I" can be used both singularly and plurally, since the subject being referred to is usually understood to be just one person. However, there are some instances where using "we" might be more appropriate, such as when referring to a group of people who identify with each other in some way (e.g., a team or family).
One possible solution could be to use the word "I-s" or "I-es" as a plural form of "I". This might not catch on widely, but it could serve as an alternative for those who feel the need for one.
Not quite the linguistic requirement though when the word requires as pluralism of the definition it.