My impression was that you only need one, and that an individual marine can be in squad mode all by himslef and gain the full benefits all alone. But the fact that it's a team-based activity seems to imply that you'd need at least two marines in squad mode to have the squad mode be effective. Anyone point me to a section of the rules that calrifies this? Thanks!

It's a minimum of two marines for squad mode. There are special exceptions such as the Killmarine advanced specialty which specifically allows a one man squad mode as a special ability. I don't have the book on me so I can't give page numbers.

As Decessor indicates, an advanced speciality exists with special abilities allowing him an exception to use certain squad mode abilities by himself.

The descriptions and rules concerning Squad Mode make it fairly obvious it is intended to work with more than one marine. Aas does the exception stated above. If there's an exception, then what the exception is excepting is the norm. In this case it normally takes two to tango (use squad mode abilities).

For Instance:

"Squad Mode abilities can only be used when the character has his fellow Battle-Brothers nearby to aid him." (Deathwatch Core, pg. 213)
-Indicates the abilities one marine activates recquires aid from fellow Battle-Brothers, who themselves must be in Squad Mode to aid or benefit from Squad Mode Abilities.

"Before a Battle-Brother can enter Squad Mode, however, he must meet the following requirements:
• He must be in Support Range of at least one other member of his Kill-team. …" (Deathwatch Core, pg. 214)
-Indicates Squad Mode cannot be activated without the presence of another Kill-team member, i.e. cannot be activated alone.

"A Battle-Brother drops out of Squad Mode—and returns to Solo Mode—at the start of his Turn if: …
• He is no longer in Support Range of at least one other member of his Kill-team." (Deathwatch Core, pg. 214)
-Indicates a Marine away from any kill-team members, i.e. no one close enough to activate squad mode, is not only unable to enter Squad Mode but is forcibly returned to Solo Mode.

"Squad Mode abilities allow a Kill-team to perform specialised combat actions, coordinated attacks, and cooperative defences in ways unavailable to lesser warriors."(Deathwatch Core, pg. 219)
-Indicates Squad Mode abiliites are carried out by more than 1 Marine, when used. You can't coordinate or cooperate with yourself.