Can an astral spotter be apart of more than one ritual spellcasting team at the same time?

no.

Why not?

The only way I can see it happen is if the two Ritual teams are targeting the same target. I would say the relationship between the spotter and the target is 1-1 but the relationship between the spotter and the Ritual team could be 1-many.

The rules aren't clear about it but the assumption being made is that you are a member of A team, not a member of multiple teams. You take drain from that ritual casting.

I can't see a way off hand of how it could be abused but given some of the creative minds here, I wouldn't be surprised to find that it can be.

I'd stick with member in one ritual team at a time.

Hm, it depends on what you mean by multiple ritual teams, too. The Great Ghost Dance, for example, had many "teams" participating in the same ritual... Does 4th edition allow spell stacking? If so, then the penalty for stacking would apply to both teams, because the spotter is doing double duty.

It has been pointed out that membership in multiple ritual castings would bring Drain to the observer from each team casting. That doesn't mean it can't be done. It's just that it -may- hurt a lot.

Ok, so:
You can only be spotting for one team at a time.
The force of your ritual casting, and the number of ritual team members is limited to the lower of the magic lodge's rating or the lowest sorcery skill level of the team.
And the team leader, though he gets to add a number of successes limited by the rating of the lodge to his total from the team, is limited to a number of successes equal to the force of the spell.
Then everybody takes drain as if they cast the spell themselves.

So...

If we have a ritual team of Josh (magic 6, sorcery 6), Sarah (magic 5, sorcery 5), and Mark (magic 4, sorcery 3) using a rating 4 level magic lodge, then the maximum force of the ritual casting would be 3 (Mark's sorcery skill), the maximum dice that Sarah and Mark could contribute to the cause would be 3 and the maximum total successes that Josh (as ritual leader) can use during the casting is 3. Then Josh, Sarah & Mark each take drain for the casting.

So why didn't Josh just cast the damned spell at force 6 by himself? That would double the number of total successes he could apply by doubling the force of the spell and then Sarah & Mark wouldn't have to take drain.

What am I missing?

That their target could be out of LOS. Edit, LOL, you mean why didn't he ritual cast by himself? I think the answer is that he would in the scenario you created. The limit on the number of members to a team being related to their sorcery is wonky.

But even if they had a level six lodge and all six members had skill level six in sorcery, the team leader still could only use 6 successes in the casting and then all six team members would have to resist drain. Why not save the drain on the other five team members and just cast it solo?

Because that's too clever. And easy.