Word for the longest time in spent in a group

Solution 1:

Really, a lot depends on the culture of the organization. While a simply titled "senior member" might be acceptable in some clubs, other possibilities, again depending on the club, could be "veteran member", "upper classman", "member emeritus", "grand poobah" (ok, maybe not the last one)

Solution 2:

If the member has been around since the group's beginnings, you can use founder or original member, depending on whether you want to emphasize a role in establishing the group.

Otherwise, you can use (most) senior member, as John Lawler suggests, or oldest member if you don't mind a little ambiguity. Eldest can also imply seniority, but it's more often used to specify the age of a family member, so you're right to avoid it.

Solution 3:

Tenure refers to the length of a person's term in a given position. So, calling a group member longest-tenured would refer to their time in the group and nothing more.

See, for example, this usage referring to a coach's length of time with a current team:

On Thursday, Charlie Manuel managed his 1,332d game with the Phillies, making him the franchise's longest-tenured manager.

Solution 4:

I think a common term for this in Britain would be longest standing member.

Oldest would normally refer to the oldest (eldest) in age. The Senior member would (if used at all), I think, be understood as referring to authority rather than tenure or age. A senior member is most likely to be understood as referring to one of a group of members over a certain age (typically 60 or 65), who may pay reduced subscriptions.

I don't think we would use longest-tenured as an adjective, and, although we would understand, member with the longest tenure, I don't think we would use it in that manner. (See also my comments under the answer with that suggestion.)