When invited to merge companies (or join another company when you already have property), you will see three options.
1: Decline - This will decline the invite.
2: Join - This will make you leave your current company, and join theirs, leaving all of your buildings, ships and tames behind. Do not choose this option. Admins will not retrieve your lost items.
3: Merge - This is the option you need. This will merge all of your current company buildings/tames/members into the new company.
Please note: If you do not select MERGE you will be able to reclaim your items/buildings/land via claim flags (approx. 10 hour timer), but will lose all of your tames. Admins will not retrieve tames due to failure to merge properly.