Firstly: The Ruling of Marriage Contracts when in Ihram
It is haram and invalid to perform a marriage contract while in ihram. This applies to the guardian, groom, and bride. There is no expiation due. This is the position of the majority: the Malikis, Shafi`is, Hanbalis, Zahiris, and a group of the Salaf.
Secondly: Proposing when in Ihram
Scholars hold two positions regarding proposing while in ihram:
The first position: It is makruh to propose when the groom or bride are in a state of ihram. It is also makruh for one in ihram to propose on behalf of those not in ihram. This is the position of the Shafi`i and Hanbali schools. This is the chosen position of Ibn Qudamah
The second position: It is haram for a man in ihram to propose. This is the position of the Maliki school. This is the chosen position of Ibn Hazm, Ibn Taymiyyah, al-Shinqiti, Ibn Baz, and Ibn `Uthaymin.
Thirdly: Being a Witness for a Marriage Contract
Being in ihram is irrelevant to witnesses of marriage contracts. This is explicitly mentioned by Shafi`i and Hanbali  Though Hanbalis view that his witnessing is makruh. jurists. This is also the conclusion of al-Shinqiti and Ibn `Uthaymin.