[ultrashogun]

The Clayden says that amidine groups are basic because the amidinium cation is stabilized by the delocalization of the positive charge over both Ns.

How can the one N participate in the conjugate system when it is sp3 hybridzed and so does not have a lone pair in a p orbital?

[Yggdrasil]

In many cases sp³ atoms with a lone pair will change their hybridization to sp² in order to place the lone pair electrons into a p orbital, allowing the lone pair to be delocalized across a conjugated pi system.

[ultrashogun]

Can I consider it a rule that of all possible hybridizations the atoms will always "choose" the one that most stabilizes the compound of which they are part?

[Yggdrasil]

That sounds about right.  For a normally sp³ atom to become sp² hybridized, it must have a lone pair.  You cannot have something bonded to four atoms be sp² hybridized.

[ultrashogun]

Cool, thanks alot.