|Hey kid, want some "boo"-tiful polish? ;)|
Solidify is on p. 151 of GURPS Magic. It can be used either by a ghost or spirit to become tangible or by a magic user to force a spirit to become tangible, and it lasts for one minute. This is almost like two separate abilities, so let's stat them separately. First the version to afflict upon oneself gives the disadvantage of not being insubstantial.
Solidify (-15%): No Insubstantiality(Sorcery -15%) [-68]
Hooray... that seems kinda unbalanced. It's purely a discount. Going with a slightly different approach, if we consider the Power-Ups approach of making an advantage out of an enhancement, we find "switchable" is a +10% enhancement, or 8 points in the case of Insubstantiality. This version is:
Solidify (-15%): Add Switchable to Insubstantiality(Sorcery -15%) 
That seems a bit more fair. Spending FP to turn it on has an automatic built in time limit of 1 minute, so for this version, we are done.
Target OthersFor the version to cast on other people, let's use the custom "No Insubstantiality" disadvantage we made up which cost the negative value of Insubtantiality, but give it a 10% discount for magical, making it [-72] We can now make it an affliction.
Solidify Other(+93%): Affliction(ST, Sorcery -15%; Disadvantage, No Insubstantiality, +8%; Malediction 1, +80%; Based on different Attribute, ST, +20%; Fixed Duration, 3 minutes, +0%) 
Optionally, you could make it based on IQ for the same cost. The spirit has a quick contest with the caster, and if it fails, it becomes substantial for 3 minutes.