Tempmem allows main owners, owners, and moderators to promote guests to member for any duration of time, up to 24 hours. Note that you must be a temporary moderator or above to use this power.
Temporary members that are signed in while their member rank expires will remain a member until they sign out and in (including changing name, avatar, and homepage). They will become a guest afterwards.
Start a private chat with the user that you want to make a temporary member and type /mb followed by the duration of time you want them to be a member for (in hours).
Example: Typing /mb12 in a user's private chat would make the user a temporary member for 12 hours.