user1799: Any way to create a multisig address from Bitcoin addresses (instead of public keys)?
If not, why not? What would the problem be?
EDITED - Thanks for the answers. This is for an application that generates multisig transactions with escrow for external users. Of course anyone who controls an address can find the public key, but that's probably beyond many casual users (or too much of an hassle).

user24925: No. OP_CHECKMULTISIG only supports multiple pubkey not pubkeyhash (addresses).

user-1: While you can't generate a multisig address directly from a number of non-multisig addresses, you can easily get their public key on a single call. In bitcoin-core you have to call validateaddress and then retrieve the pubkey field from the response, which is the hex value of the raw public key that you can then use to generate your multisig.

user2306: This is possible, but there isn't really any code to do it automatically.
Instead of creating a scriptPubKey like 
<pubkey 1> <pubkey 2> <pubkey 3> 2 OP_CHECKSIG

you'd make one like
OP_DUP OP_HASH160 <pubkeyhash 1> OP_EQUALVERIFY OP_ROT
OP_DUP OP_HASH160 <pubkeyhash 2> OP_EQUALVERIFY OP_ROT 
OP_DUP OP_HASH160 <pubkeyhash 3> OP_EQUALVERIFY OP_ROT 2 OP_CHECKSIG

Also, the code of the receiver would need to change, in order to recognize the output as spendable.
It's probably much easier to get their public keys. If they've used their addresses on the network, you can look it up using https://blockchain.info/q/pubkeyaddr/<addr>
Example: https://blockchain.info/q/pubkeyaddr/1snowqQP5VmZgU47i5AWwz9fsgHQg94Fa
Or, you can use George Kimionis's method to decode addresses.

1799: <p>Any way to create a multisig address from Bitcoin addresses (instead of public keys)?</p>

<p>If not, why not? What would the problem be?</p>

<p>EDITED - Thanks for the answers. This is for an application that generates multisig transactions with escrow for external users. Of course anyone who controls an address can find the public key, but that's probably beyond many casual users (or too much of an hassle).</p>
 24925: <p>No. <code>OP_CHECKMULTISIG</code> only supports multiple pubkey not pubkeyhash (addresses).</p>
 -1: <p>While you can't generate a multisig address directly from a number of non-multisig addresses, you can easily get their public key on a single call. In <code>bitcoin-core</code> you have to call <code>validateaddress</code> and then retrieve the <code>pubkey</code> field from the response, which is the hex value of the raw public key that you can then use to generate your multisig.</p>
 2306: <p>This is possible, but there isn't really any code to do it automatically.</p>

<p>Instead of creating a scriptPubKey like </p>

<pre><code>&lt;pubkey 1&gt; &lt;pubkey 2&gt; &lt;pubkey 3&gt; 2 OP_CHECKSIG
</code></pre>

<p>you'd make one like</p>

<pre><code>OP_DUP OP_HASH160 &lt;pubkeyhash 1&gt; OP_EQUALVERIFY OP_ROT
OP_DUP OP_HASH160 &lt;pubkeyhash 2&gt; OP_EQUALVERIFY OP_ROT 
OP_DUP OP_HASH160 &lt;pubkeyhash 3&gt; OP_EQUALVERIFY OP_ROT 2 OP_CHECKSIG
</code></pre>

<p>Also, the code of the receiver would need to change, in order to recognize the output as spendable.</p>

<p>It's probably much easier to get their public keys. If they've used their addresses on the network, you can look it up using <code>https://blockchain.info/q/pubkeyaddr/&lt;addr&gt;</code></p>

<p>Example: <a href="https://blockchain.info/q/pubkeyaddr/1snowqQP5VmZgU47i5AWwz9fsgHQg94Fa" rel="nofollow">https://blockchain.info/q/pubkeyaddr/1snowqQP5VmZgU47i5AWwz9fsgHQg94Fa</a></p>

<p>Or, you can use George Kimionis's method to decode addresses.</p>
